#include<bits/stdc++.h>
using namespace std;
template<class T> using matrix = vector<vector<T>>;
int n;
vector<matrix<int>> pref;
void init(string a, string b){
n = a.size();
for(int i = 0; i < n; i ++){
int A = (a[i] == 'A' ? 0 : (a[i] == 'T' ? 1 : 2));
int B = (b[i] == 'A' ? 0 : (b[i] == 'T' ? 1 : 2));
if(pref.empty()) pref.push_back(matrix<int>(3, vector<int>(3)));
else pref.push_back(pref.back());
++ pref.back()[A][B];
}
}
int get_distance(int x, int y){
matrix<int> left = (x ? pref[x - 1] : matrix<int>(3, vector<int>(3, 0)));
matrix<int> right = pref[y];
matrix<int> diff(3, vector<int>(3, 0));
for(int i = 0; i < 3; i ++){
for(int j = 0; j < 3; j ++){
diff[i][j] = right[i][j] - left[i][j];
}
}
int ans = 0;
for(int i = 0; i < 3; i ++) for(int j = i + 1; j < 3; j ++){
int t = min(diff[i][j], diff[j][i]);
ans += t;
diff[i][j] -= t;
diff[j][i] -= t;
}
// 0 > 1 > 2
{
int t = min({diff[0][1], diff[1][2], diff[2][0]});
ans += 2 * t;
diff[0][1] -= t;
diff[1][2] -= t;
diff[2][0] -= t;
}
// 0 > 2 > 1
{
int t = min({diff[0][2], diff[2][1], diff[1][0]});
ans += 2 * t;
diff[0][2] -= t;
diff[2][1] -= t;
diff[1][0] -= t;
}
int excess = 0;
for(int i = 0; i < 3; i ++){
for(int j = 0; j < 3; j ++){
if(i == j) continue;
excess += diff[i][j];
}
}
return (excess ? -1 : ans);
}
/*
int main(){
string a, b; cin >> a >> b;
init(a, b);
while(true){
int x, y; cin >> x >> y;
cout << get_distance(x, y) << endl;
}
}
*/
