#include <bits/stdc++.h>
long int temp, i, j, k, T;
long int ABS(int test) {
  if (test < 0)
    return test * -1;
  else
    return test;
}
int x8[] = {1, 1, 0, -1, -1, -1, 0, 1};
int y8[] = {0, 1, 1, 1, 0, -1, -1, -1};
int x4[] = {1, 0, -1, 0};
int y4[] = {0, 1, 0, -1};
long int arr[1000000];
long int buff[1000000];
long int memo[1000000];
using namespace std;
int main() {
  char a[100010];
  char b[100010];
  int jawaban = 0, panjang, tujuh1 = 0, tujuh2 = 0, empat1 = 0, empat2 = 0;
  memset(memo, 0, sizeof memo);
  cin >> a;
  cin >> b;
  panjang = strlen(a);
  for (i = 0; i < panjang; i++) {
    if (a[i] == '7') tujuh1++;
    if (a[i] == '4') empat1++;
    if (b[i] == '7') tujuh2++;
    if (b[i] == '4') empat2++;
  }
  for (i = 0; i < panjang; i++) {
    if (a[i] == b[i]) {
      if (a[i] == '7') {
        tujuh1--;
        tujuh2--;
      }
      if (a[i] == '4') {
        empat1--;
        empat2--;
      }
    }
  }
  jawaban = max(tujuh1, tujuh2);
  cout << jawaban << endl;
  {
    fflush(stdin);
    getchar();
  };
  return 0;
}
