#include <bits/stdc++.h>
using namespace std;

typedef long long ll;
typedef double lf;
typedef pair <int, int> pii;
typedef pair <ll, ll> pll;

#define TRACE(x) cerr << #x << ' ' << x << endl
#define FOR(i, a, b) for (int i = (a); i < (b); i++)
#define REP(i, n) FOR(i, 0, n)
#define _ << ' ' <<

#define fi first
#define sec second
#define se second
#define mp make_pair
#define pb push_back

int trans(char c) {
  if (c == 'A') return 0;
  if (c == 'C') return 1;
  if (c == 'G') return 2;
  if (c == 'T') return 3;
  return -1;
}

int cnt[4][4];

int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);

  int n;
  string s, t;
  cin >> s >> t;
  n = s.length();
  int B = 0;
  for (int i = 0; i < n; i++) {
    if (s[i] == t[i]) B++;
    else cnt[trans(s[i])][trans(t[i])]++;
  }

  int A = 0;
  vector<vector<int>> V;
  for (int i = 0; i < 4; i++)
    for (int j = 0; j < 4; j++) {
      if (i == j) continue;
      V.push_back({i, j});
    }
  for (int i = 0; i < 4; i++)
    for (int j = 0; j < 4; j++) {
      if (i == j) continue;
      for (int k = 0; k < 4; k++) {
        if (k == i || k == j) continue;
        V.push_back({i, j, k});
      }
    }

  for (auto v : V) {
    vector<pair<int, int>> q;
    for (int i = 0; i < (int)v.size(); i++)
      q.push_back({v[i], v[(i + 1) % (int)v.size()]});
    while (true) {
      bool ok = true;
      for (auto p : q) ok &= (cnt[p.fi][p.sec] > 0);
      if (ok) {
        A++;
        for (auto p : q) ok &= cnt[p.fi][p.sec]--;
      } else {
        break;
      }
    }
  }

  int C = 0;
  for (int i = 0; i < 4; i++)
    for (int j = 0; j < 4; j++) 
      if (i != j) C += cnt[i][j];
  assert(C % 4 == 0);
  C /= 4;

  cout << n - A - B - C;


  return 0;
}