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

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

#define pb push_back
#define x first
#define y second

#define LT <
#define GT >

typedef long long ll;
typedef long double lf;

const int MAXN = 1001000;
char s[MAXN];
char w[MAXN];

int G[5][5];

bool bio[5];

vector<int> cyc;

void dfs(int x) {
  if (bio[x]) return;
  bio[x] = true;
  cyc.push_back(x);
  REP(y, 4) {
    if (x == y || G[x][y] == 0) continue;
    dfs(y);
    break;
  }
}

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

bool can(int x) {
  REP(i, 4)
    if (G[x][i] > 0)
      return true;
  return false;
}

int main(void) {
  scanf("%s", s);
  scanf("%s", w);
  const int sz = strlen(s);
  REP(i, sz) {
    int x = kod(s[i]);
    int y = kod(w[i]);
    if (x == y) continue;
    G[x][y]++;
  }

  int ans = 0;
  REP(i, 4) {
    REP(j, i) {
      int k = min(G[i][j], G[j][i]);
      ans += k;
      G[i][j] -= k;
      G[j][i] -= k;
    }
  }

  REP(iter, 20) {
    memset(bio, 0, sizeof bio);
    REP(i, 4) {
      if (!can(i)) continue;
      cyc.clear();
      dfs(i);
      if (cyc.size() < 2u) continue;
      int k = G[cyc[0]][cyc[1]];
      REP(j, cyc.size()) {
        int jn = (j + 1) % (int)cyc.size();
        k = min(k, G[cyc[j]][cyc[jn]]);
      }
      REP(j, cyc.size()) {
        int jn = (j + 1) % (int)cyc.size();
        G[cyc[j]][cyc[jn]] -= k;
      }

      ans += (int)(cyc.size() - 1) * k;
    }
  }

  printf("%d\n", ans);

  return 0;
}