#include <bits/stdc++.h>
using namespace std;
using ll=long long;
using ld=double;
#define FOR(i,a,b) for(ll i=a;i<(ll)b;++i)
#define F(n) FOR(i,0,n)
#define FF(n) FOR(j,0,n)
#define aa first
#define bb second
#define PB push_back
#define EQ(a,b) (fabs(a-b)<=(fabs(a+b)*EPS))
#define MOD ((ll)(1e9+7))
#define INF (1ll << 40)

ll mtx[4][4];
int main() {
  ios::sync_with_stdio(0);cin.tie(0);
  string a, b;
  cin >> a >> b;
  ll n = a.size();
  map<char,ll> m = {{'A', 0}, {'C', 1}, {'G', 2}, {'T', 3}};

  F(n) {
    mtx[m[a[i]]][m[b[i]]] ++;
  }
  //cout << m['A'] << endl;
  // cycles 1
  ll res = 0;
  F(4) mtx[i][i] = 0;
  // 2
  F(4) {
    FF(4) {
      if (i == j) continue;
      ll c = min(mtx[i][j], mtx[j][i]);
      mtx[i][j] -= c;
      mtx[j][i] -= c;
      res += c;
    }
  }
  // 3
  F(4) {
    FF(4) {
      FOR(k,0,4) {
        if (i == j || j == k || i == k) continue;
        ll c = min(mtx[i][j], min(mtx[j][k], mtx[k][i]));
        mtx[i][j] -= c;
        mtx[j][k] -= c;
        mtx[k][i] -= c;
        res += 2*c;
      }
    }
  }
  ll sum = 0;
  F(4) FF(4) sum += mtx[i][j];
  res += (sum/4)*3;
  cout << res << endl;

  return 0;
}