#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define vb vector<bool>
#define pb push_back
#define ff(aa, bb, cc) for(ll aa = bb; aa < cc; aa++)
#define vl vector<ll>
#define pll pair<ll, ll>
#define fi first
#define se second
#define ed "\n"
#define all(aaa) aaa.begin(), aaa.end()
#define rall(aaa) aaa.rbegin(), aaa.rend()
string a, b;
vl aa, ac, at, ba, bc, bt;
vl AA, AC, AT, CA, CC, CT, TA, TC, TT;
void init(string x, string y){
a = x;
b = y;
ll n = a.size();
aa = vl(n, 0);
ac = vl(n, 0);
at = vl(n, 0);
ba = vl(n, 0);
bc = vl(n, 0);
bt = vl(n, 0);
AA = vl(n, 0);
AC = vl(n, 0);
AT = vl(n, 0);
CA = vl(n, 0);
CC = vl(n, 0);
CT = vl(n, 0);
TA = vl(n, 0);
TC = vl(n, 0);
TT = vl(n, 0);
ff(i, 0, n){
if(i != 0){
aa[i] = aa[i-1];
ac[i] = ac[i-1];
at[i] = at[i-1];
ba[i] = ba[i-1];
bc[i] = bc[i-1];
bt[i] = bt[i-1];
AA[i] = AA[i-1];
AC[i] = AC[i-1];
AT[i] = AT[i-1];
CA[i] = CA[i-1];
CC[i] = CC[i-1];
CT[i] = CT[i-1];
TA[i] = TA[i-1];
TC[i] = TC[i-1];
TT[i] = TT[i-1];
}
if(a[i] == 'A'){
aa[i]++;
}
else if(a[i] == 'C'){
ac[i]++;
}
else{
at[i]++;
}
if(b[i] == 'A'){
ba[i]++;
}
else if(b[i] == 'C'){
bc[i]++;
}
else{
bt[i]++;
}
if(a[i] == 'A' && b[i] == 'A'){
AA[i]++;
}
if(a[i] == 'A' && b[i] == 'C'){
AC[i]++;
}
if(a[i] == 'A' && b[i] == 'T'){
AT[i]++;
}
if(a[i] == 'C' && b[i] == 'A'){
CA[i]++;
}
if(a[i] == 'C' && b[i] == 'C'){
CC[i]++;
}
if(a[i] == 'C' && b[i] == 'T'){
CT[i]++;
}
if(a[i] == 'T' && b[i] == 'A'){
TA[i]++;
}
if(a[i] == 'T' && b[i] == 'C'){
TC[i]++;
}
if(a[i] == 'T' && b[i] == 'T'){
TT[i]++;
}
}
}
int get_distance(int x, int y){
ll qaa = aa[y], qac = ac[y], qat = at[y];
ll qba = ba[y], qbc = bc[y], qbt = bt[y];
ll qAA = AA[y], qAC = AC[y], qAT = AT[y];
ll qCA = CA[y], qCC = CC[y], qCT = CT[y];
ll qTA = TA[y], qTC = TC[y], qTT = TT[y];
if(x != 0){
qaa = aa[y]-aa[x-1];
qac = ac[y]-ac[x-1];
qat = at[y]-at[x-1];
qba = ba[y]-ba[x-1];
qbc = bc[y]-bc[x-1];
qbt = bt[y]-bt[x-1];
qAA = AA[y]-AA[x-1];
qAC = AC[y]-AC[x-1];
qAT = AT[y]-AT[x-1];
qCA = CA[y]-CA[x-1];
qCC = CC[y]-CC[x-1];
qCT = CT[y]-CT[x-1];
qTA = TA[y]-TA[x-1];
qTC = TC[y]-TC[x-1];
qTT = TT[y]-TT[x-1];
}
if(qaa != qba || qac != qbc || qat != qbt){
//cout << qaa << " " << qba << ed;
//cout << qac << " " << qbc << ed;
//cout << qat << " " << qbt << ed;
return -1;
}
int ans = 0;
int aux = min(qAC, qCA);
ans += aux;
qAC -= aux;
qCA -= aux;
//cout << "CA" << aux << ed;
aux = min(qAT, qTA);
ans += aux;
qAT -= aux;
qTA -= aux;
//cout << "AT" << aux << ed;
aux = min(qCT, qTC);
ans += aux;
qTC -= aux;
qCT -= aux;
//cout << "CT" << aux << ed;
//cout << "TOT " << res << " " << aux << ed;
//cout << wrong[y] << " " << wrong[x-1] << ed;
int ex = qAT+qTA+qCA+qAC+qCT+qTC;
ans += (ex/3)*2;
return ans;
}
/*
int main(){
ll n, q;
cin >> n >> q;
string A, B;
cin >> A >> B;
init(A, B);
while(q--){
int na, nb;
cin >> na >> nb;
cout << get_distance(na, nb) << ed;
}
}
*/
