#include <stdio.h>

static char P[1000004];
static char Q[1000004];
static int R[4][4];

static int genei(char c) {
    if (c == 'A') return 0;
    if (c == 'C') return 1;
    if (c == 'G') return 2;
    return 3;
}

static int getmin(void) {
    int i = 0, mi = -1;
    for (; i < 16; i++) {
        if (!R[i>>2][i&3]) continue;
        if (mi == -1 || R[i>>2][i&3] < R[mi>>2][mi&3]) mi = i;
    }
    return mi;
}

static int dfs(int s, int e, int d) {
    if (d <= 0) return -1;
    if (s == e) return 0;
    int i = 0, m = -1;
    for (; i < 4; i++) {
        if (!R[s][i]) continue;
        int l = dfs(i, e, d-1);
        if (l < 0) continue;
        l = l<<4 | (s<<2) | i;
        if (m != -1 && l > m) continue;
        m = l;
    }
    return m;
}

int main() {
    int i = 0;
    long long r = 0;
    scanf("%s %s", P, Q);
    for (; P[i] && Q[i]; i++) {
        int p = genei(P[i]);
        int q = genei(Q[i]);
        if (p != q) R[p][q]++;
    }
    while (1) {
        int mi = getmin();
        if (mi < 0) break;
        int vi = R[mi>>2][mi&3];
        R[mi>>2][mi&3] = 0;
        int l = dfs(mi&3, mi>>2, 4);
        if (l < 0) return -1;
        for (i = 0; i < 16; i += 4) {
            int li = (l >> i) & 15;
            if ((li>>2) != (li&3)) {
                R[li>>2][li&3] -= vi;
                r += vi;
            }
        }
    }
    printf("%lld\n", r);
    return 0;
}