C++ exercises: read file with fstream and count punctuation marks

Q: Write a program that reads a text file and prints a count of the number of occurrences of the punctuation marks (.,!?).

#include <iostream>
#include <fstream>
#include <string>
 
using namespace std;
 
int main(int argc, char *argv[])
{
   if (argc < 2)
   {
      cout << "Usage: " << argv[0] << " <input_file>" << endl;
      return -1;
   }
 
   string s;
   string chs = ",.!?";
   int count = 0;
 
   fstream f(argv[1]);
   while (f)
   {
      getline(f, s);
 
      for (unsigned int i = 0; i < s.size(); i++)
      {
         if (chs.find(s[i]) != string::npos)
         {
            count++;
         }
      }
   }
 
   cout << "count: " << count << endl;

   return 0;
}