/*
Zadanie: Mutating DNA
Autor: Tomasz Kwiatkowski
*/
#include <bits/stdc++.h>
#define fi first
#define se second
#define pb push_back
using namespace std;
typedef long long ll;
const int MAXN = 1000*1000 + 7;
const int INF = 1000*1000*1000 + 7;
char toChar[3] = {'A', 'T', 'C'};
int cnt1[MAXN][3], cnt2[MAXN][3];
int pref[MAXN][3][3];
void init(string a, string b)
{
int n = a.size();
for (int i = 1; i <= n; ++i) {
for (int c1 = 0; c1 < 3; ++c1) {
cnt1[i][c1] = cnt1[i - 1][c1];
cnt2[i][c1] = cnt2[i - 1][c1];
if (a[i - 1] == toChar[c1])
++cnt1[i][c1];
if (b[i - 1] == toChar[c1])
++cnt2[i][c1];
for (int c2 = 0; c2 < 3; ++c2) {
pref[i][c1][c2] = pref[i - 1][c1][c2];
if (a[i - 1] == toChar[c1] && b[i - 1] == toChar[c2]) {
++pref[i][c1][c2];
}
}
}
}
}
int get_distance(int x, int y)
{
++x, ++y;
for (int c = 0; c < 3; ++c) {
if ((cnt1[y][c] - cnt1[x - 1][c]) != (cnt2[y][c] - cnt2[x - 1][c])) {
return -1;
}
}
int tmp[3][3];
for (int c1 = 0; c1 < 3; ++c1) {
for (int c2 = 0; c2 < 3; ++c2) {
tmp[c1][c2] = pref[y][c1][c2] - pref[x - 1][c1][c2];
}
}
int res = 0;
int minn = min(tmp[0][1], tmp[1][0]);
tmp[0][1] -= minn;
tmp[1][0] -= minn;
res += minn;
minn = min(tmp[0][2], tmp[2][0]);
tmp[0][2] -= minn;
tmp[2][0] -= minn;
res += minn;
minn = min(tmp[2][1], tmp[1][2]);
tmp[2][1] -= minn;
tmp[1][2] -= minn;
res += minn;
int sum = 0, maxx = 0;
for (int c1 = 0; c1 < 3; ++c1) {
for (int c2 = 0; c2 < 3; ++c2) {
sum += tmp[c1][c2];
maxx = max(tmp[c1][c2], maxx);
}
}
res += (sum - maxx);
return res;
}
/*int main()
{
ios_base::sync_with_stdio(0), cin.tie(0);
int n, q;
cin >> n >> q;
string s1, s2;
cin >> s1 >> s2;
init(s1, s2);
while (q--) {
int x, y;
cin >> x >> y;
cout << get_distance(x, y) << '\n';
}
return 0;
}
*/
