//#include "dna.h"
#include <bits/stdc++.h>
using namespace std;
int n,ct[5][100005][5],a[5][5],num,num2,tmp,ans;
int ps[100005][5][5];
char s[8]={'A','C','T'};
void init(std::string a, std::string b) {
n=a.length();
for (int i=0;i<n;i++){
for (int j=0;j<3;j++){
if (a[i] == s[j]) ct[1][i][j]++;
if (b[i] == s[j]) ct[2][i][j]++;
}
for (int j=0;j<3;j++){
for (int k=0;k<3;k++){
if (a[i] == s[j] && b[i] == s[k])
ps[i][j][k]++;
}
}
if (i == 0) continue;
for (int j=0;j<3;j++){
for (int k=0;k<3;k++){
ps[i][j][k]+=ps[i-1][j][k];
}
}
for (int j=0;j<3;j++){
ct[1][i][j]+=ct[1][i-1][j];
ct[2][i][j]+=ct[2][i-1][j];
}
}
}
int get_distance(int x, int y) {
ans=0;
for (int i=0;i<3;i++){
if (x == 0 && ct[1][y][i] != ct[2][y][i])
return -1;
if (x != 0 && ct[1][y][i]-ct[1][x-1][i] != ct[2][y][i]-ct[2][x-1][i])
return -1;
}
for (int i=0;i<3;i++){
for (int j=0;j<3;j++){
if (x == 0) a[i][j]=ps[y][i][j];
else a[i][j]=ps[y][i][j]-ps[x-1][i][j];
}
}
for (int i=0;i<3;i++){
for (int j=0;j<3;j++){
tmp=min(a[i][j],a[j][i]);
if (i != j) ans+=tmp;
a[i][j]-=tmp;
if (i != j) a[j][i]-=tmp;
}
}
for (int i=0;i<3;i++){
for (int j=0;j<3;j++){
if (a[i][j] != 0)
return a[i][j]*2+ans;
}
}
return ans;
}
/*
string r,g;
int aa,bb;
int main(){
cin >> r >> g;
init(r,g);
for (int i=1;i<=4;i++){
cin >> aa >> bb;
cout << get_distance(aa,bb) << "\n";
}
}*/
