#include <stdio.h>

static char P[1000004];
static char Q[1000004];
static long long 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 long long min(long long x, long long y) {
    return x < y ? x : y;
}

static long long cycle2(int x, int y) {
    long long v = min(R[x][y], R[y][x]);
    R[x][y] -= v;
    R[y][x] -= v;
    return v;
}

static long long cycle3(int x, int y, int z) {
    long long v = min(min(R[x][y], R[y][z]), R[z][x]);
    R[x][y] -= v;
    R[y][z] -= v;
    R[z][x] -= v;
    return v;
}

static long long cycle4(int x, int y, int z, int w) {
    long long v = min(min(R[x][y], R[y][z]), min(R[z][w], R[w][x]));
    R[x][y] -= v;
    R[y][z] -= v;
    R[z][w] -= v;
    R[w][x] -= v;
    return v;
}

int main() {
    int i = 0, j = 0, k = 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]++;
    }
    for (i = 0; i < 4; i++) {
        for (j = 0; j < 4; j++) {
            if (i != j) r += cycle2(i, j);
        }
    }
    for (i = 0; i < 4; i++) {
        for (j = 0; j < 4; j++) {
            if (i == j) continue;
            for (k = 0; k < 4; k++) {
                if (i != k && j != k) r += 2 * cycle3(i, j, k);
            }
        }
    }
    for (i = 0; i < 4; i++) {
        for (j = 0; j < 4; j++) {
            if (i == j) continue;
            for (k = 0; k < 4; k++) {
                if (i != k && j != k) r += 3 * cycle4(i, j, k, 6-i-j-k);
            }
        }
    }
    printf("%lld\n", r);
    return 0;
}