#include <bits/stdc++.h>
int DEBUG = 0;
int Max(int a, int b) { return (a > b ? a : b); }
using namespace std;
int main(int argc, char **argv) {
  char a[1000000], b[1000000];
  cin >> a >> b;
  int len = strlen(a);
  int s1 = 0;
  int s2 = 0;
  for (int i = 0; i < len; i++) {
    if (a[i] == b[i]) {
    } else if (a[i] < b[i]) {
      s1++;
    } else {
      s2++;
    }
  }
  cout << Max(s1, s2) << endl;
  return 0;
}
