Program crashes after clicking button

package com.yoursrishabh.calculator;

import androidx.appcompat.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity
{

    EditText first,second;
    Button addB,subB,mulB,divB;
    TextView res;

    @Override
    protected void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        first = (EditText)findViewById(R.id.tf1);
        second = (EditText)findViewById(R.id.tf2);
        addB = (Button)findViewById(R.id.btnAdd);
        subB = (Button)findViewById(R.id.btnSub);
        mulB = (Button)findViewById(R.id.btnMul);
        divB = (Button)findViewById(R.id.btnDiv);
        res = (TextView)findViewById(R.id.tvRes);

        addB.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v)
            {
                int f = Integer.parseInt(first.getText().toString());
                int s = Integer.parseInt(second.getText().toString());
                int sum = f+s;
                res.setText(sum);
            }
        });

        subB.setOnClickListener(new View.OnClickListener(){
            @Override
            public void onClick(View v) {
                int f = Integer.parseInt(first.getText().toString());
                int s = Integer.parseInt(second.getText().toString());
                int sub = f-s;
                res.setText(sub);
            }
        });


    }
}

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:visibility="visible"
    tools:context=".MainActivity"
    tools:visibility="visible">

    <EditText
        android:id="@+id/pt1"
        android:layout_width="150dp"
        android:layout_height="wrap_content"
        android:layout_marginStart="25dp"
        android:layout_marginTop="30dp"
        android:autofillHints=""
        android:ems="10"
        android:inputType="textPersonName"
        android:text="First no."
        android:textColor="#E32D2D"
        android:textSize="22sp"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <EditText
        android:id="@+id/pt2"
        android:layout_width="180dp"
        android:layout_height="wrap_content"
        android:layout_marginStart="25dp"
        android:layout_marginTop="20dp"
        android:autofillHints=""
        android:ems="10"
        android:inputType="textPersonName"
        android:text="Second no."
        android:textAlignment="viewStart"
        android:textColor="#E32D2D"
        android:textSize="22sp"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/pt1" />

    <com.google.android.material.textfield.TextInputEditText
        android:id="@+id/tf1"
        android:layout_width="120dp"
        android:layout_height="wrap_content"
        android:layout_marginStart="70dp"
        android:layout_marginTop="36dp"
        android:hint="Enter no."
        android:visibility="visible"
        app:layout_constraintStart_toEndOf="@+id/pt1"
        app:layout_constraintTop_toTopOf="parent" />

    <com.google.android.material.textfield.TextInputEditText
        android:id="@+id/tf2"
        android:layout_width="120dp"
        android:layout_height="53dp"
        android:layout_marginStart="40dp"
        android:layout_marginTop="20dp"
        android:hint="Enter no."
        app:layout_constraintStart_toEndOf="@+id/pt2"
        app:layout_constraintTop_toBottomOf="@+id/tf1" />

    <Button
        android:id="@+id/btnAdd"
        android:layout_width="77dp"
        android:layout_height="58dp"
        android:layout_marginStart="49dp"
        android:layout_marginTop="72dp"
        android:background="#CDDC39"
        android:text="Add"
        android:textColor="#FFFFFF"
        android:textSize="17sp"
        android:textStyle="bold"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/pt2" />

    <TextView
        android:id="@+id/tvRes"
        android:layout_width="120dp"
        android:layout_height="30dp"
        android:layout_marginStart="48dp"
        android:layout_marginTop="304dp"
        android:text="Answer:"
        android:textColor="#F44336"
        android:textSize="20sp"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/btnAdd" />

    <Button
        android:id="@+id/btnSub"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="70dp"
        android:layout_marginEnd="118dp"
        android:text="Sub"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/tf2" />

    <Button
        android:id="@+id/btnMul"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="36dp"
        android:layout_marginEnd="111dp"
        android:layout_marginBottom="160dp"
        android:text="Mult"
        app:layout_constraintBottom_toTopOf="@+id/tvRes"
        app:layout_constraintEnd_toStartOf="@+id/btnDiv"
        app:layout_constraintStart_toStartOf="parent" />

    <Button
        android:id="@+id/btnDiv"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginEnd="88dp"
        android:layout_marginBottom="305dp"
        android:text="Div"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent" />

</androidx.constraintlayout.widget.ConstraintLayout>