#include <bits/stdc++.h>
using namespace std;
#pragma GCC optimize("Ofast")
#pragma GCC optimize("unroll-loops")
#pragma GCC optimize("-ffloat-store")
#pragma GCC optimize("-fno-defer-pop")
long long int power(long long int a, long long int b, long long int m) {
  if (b == 0) return 1;
  if (b == 1) return a % m;
  long long int t = power(a, b / 2, m) % m;
  t = (t * t) % m;
  if (b & 1) t = ((t % m) * (a % m)) % m;
  return t;
}
long long int modInverse(long long int a, long long int m) {
  return power(a, m - 2, m);
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  char s1[100001];
  char s2[100001];
  cin >> s1;
  cin >> s2;
  long long int i, j, k, l, n = strlen(s1);
  k = 0;
  l = 0;
  for (i = 0; i < n; i++) {
    if (s1[i] != s2[i]) {
      if (s2[i] - '0' == 4)
        k++;
      else
        l++;
    }
  }
  j = min(k, l);
  k = k - j;
  l = l - j;
  cout << j + k + l;
  return 0;
}
