#include <bits/stdc++.h>
using namespace std;
int main() {
  string a, b;
  int k = 0, l = 0, n;
  cin >> a >> b;
  n = a.size();
  for (int i = 0; i < n; i++) {
    if (a[i] == '4' && b[i] == '7') {
      k++;
    } else if (a[i] == '7' && b[i] == '4') {
      l++;
    }
  }
  int p = max(k, l);
  cout << p << endl;
  return 0;
}
