#include <bits/stdc++.h>
using namespace std;
const int MAX = 1e5 + 5;
char a[MAX], b[MAX];
int main(void) {
  scanf("%s %s", a, b);
  int len = strlen(a);
  int cnt[2];
  cnt[0] = cnt[1] = 0;
  for (int i = 0; i < len; ++i)
    cnt[0] += (a[i] < b[i]), cnt[1] += (a[i] > b[i]);
  if (cnt[0] > cnt[1]) swap(cnt[0], cnt[1]);
  printf("%d", cnt[0] + cnt[1] - cnt[0]);
  return 0;
}
