#include <bits/stdc++.h>
#pragma GCC optimize("Ofast")
#pragma GCC target("avx,avx2,fma")
#pragma comment(linker, "/stack:200000000")
#pragma GCC optimize("unroll-loops")
using namespace std;
template <typename A>
ostream &operator<<(ostream &cout, vector<A> const &v);
template <typename A, typename B>
ostream &operator<<(ostream &cout, pair<A, B> const &p) {
  return cout << "(" << p.first << ", " << p.second << ")";
}
template <typename A>
ostream &operator<<(ostream &cout, vector<A> const &v) {
  cout << "[";
  for (int i = 0; i < v.size(); i++) {
    if (i) cout << ", ";
    cout << v[i];
  }
  return cout << "]";
}
template <typename A, typename B>
istream &operator>>(istream &cin, pair<A, B> &p) {
  cin >> p.first;
  return cin >> p.second;
}
const long long MAXN = 1e5 + 7;
void check() {
  long long a[1000] = {0}, b[1000] = {0}, ans = 0;
  string s1, s2;
  cin >> s1 >> s2;
  for (long long i = 0; i < s1.size(); i++) a[s1[i]]++, b[s2[i]]++;
  for (long long i = 0; i < s1.size(); i++) {
    if (s2[i] != s1[i]) {
      if (a[s2[i]] < b[s2[i]]) {
        ans++;
        a[s2[i]]++;
        a[s1[i]]--;
        s1[i] = s2[i];
      }
    }
  }
  long long cnt = 0;
  for (long long i = 0; i < s1.size(); i++) cnt += (s1[i] != s2[i]);
  cnt /= 2;
  cout << ans + cnt;
}
int32_t main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int t = 1;
  for (int i = 1; i <= t; i++) {
    check();
  }
  return 0;
}
