Operator '-' cannot be applied to operands 'string' and 'int'

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace inClass3
{
    class Program
    {
        static void Main(string[] args)
        {

            //Declare & Initalize Variables
            string firstNum = "";
            string secondNum = "";
            string thirdNum = "";
            string fourthNum = "";
            string fifthNum = "";

            //Get user input 
            Console.Write("Please enter the first number (has to be two digits): ");
            firstNum = Console.ReadLine();
            
            Console.Write("Please enter the second number (has to be two digits): ");
            secondNum = Console.ReadLine();

            Console.Write("Please enter the third number (has to be two digits): ");
            thirdNum = Console.ReadLine();

            Console.Write("Please enter the fourth number (has to be two digits): ");
            fourthNum = Console.ReadLine();

            Console.Write("Please enter the fifth number (has to be two digits): ");
            fifthNum = Console.ReadLine();


            //Convert all string inputs to double
            Console.WriteLine(Convert.ToDouble(firstNum));
            Console.WriteLine(Convert.ToDouble(secondNum));
            Console.WriteLine(Convert.ToDouble(thirdNum));
            Console.WriteLine(Convert.ToDouble(fourthNum));
            Console.WriteLine(Convert.ToDouble(fifthNum));


            //Adding first four numbers in a seperate variable
            int firstFour = int.Parse(firstNum + secondNum + thirdNum + fourthNum);

            //Add the last two numbers and stored inside a new variable 
            int lastTwo = int.Parse(firstFour + fifthNum);

            //Multiply firstFour variable & lastTwo variable and store it in a new variable 
            int multiplyTwoSums = (firstFour * lastTwo);

            //Subtract fifth number from first four variables (firstFour) 
            int allNums = int.Parse(fifthNum - firstFour); **Problem is here**





            Console.ReadKey();
        }
    }
}