#include <bits/stdc++.h>
using namespace std;
map<long long, bool> tz;
int gcd(int a, int b) {
  if (a == 0)
    return b;
  else
    return gcd(b, b % a);
}
int cnk(long long n, long long k, long long mod) {
  long long result = 1;
  int lal = 0;
  for (int lol = n; lal < k; lal++, lol--) {
    result *= lol;
    result %= mod;
  }
  return result;
}
string s;
const long long mod = 1e9 + 7;
vector<long long> graph[100000];
vector<long long> vec1, vec2, vec3;
vector<pair<long long, long long> > vec;
long long res;
long long mas[10][10];
int main() {
  string s1, s2;
  cin >> s1 >> s2;
  long long kol1 = 0, kol2 = 0;
  for (long long lol = 0; lol < s1.length(); lol++) {
    if (s1[lol] > s2[lol]) kol1++;
    if (s1[lol] < s2[lol]) kol2++;
  }
  cout << (max(kol1, kol2));
  return 0;
}
