#include <bits/stdc++.h>
int main() {
  char s[100005], t[100005];
  int i;
  int different = 0;
  int more = 0;
  gets(s);
  gets(t);
  for (i = 0; s[i] != '\0'; i++) {
    if (s[i] > t[i]) {
      more++;
      different++;
    } else if (s[i] < t[i]) {
      more--;
      different++;
    }
  }
  if (more < 0) more *= -1;
  printf("%d\n", (more + (different - more) / 2));
  return 0;
}
