How to calculate the length of the shortest word in this file (a word is a group of characters that are consecutive and different from spaces and punctuation)

#include <iostream>
#include <fstream>
#include <string>

using namespace std;

int main()
{
    const string PUNC = " .,:;!?-'";
	char fname[100]; // file name
	cout << "enter file name: "; 
	cin >> fname;

	string shortest, longest, word;
	ifstream fin(fname); // opened the file for reading
	fin >> word;
	shortest = longest = word;
	while (getline(fin, word)) {
		if(word.size() > longest.size())
			longest = word;
		else if (word.size() < shortest.size())
			shortest = word;
	}

	cout << "The longest " << longest << " with a lenght " << longest.size() << endl;
	cout << "The shortest " << shortest << " with a lenght " << shortest.size() << endl;
	return 0;
}