#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
template<typename T>
bool assign_min(T& a, T b) {
if (a > b) {
a = b;
return true;
}
return false;
}
template<typename T>
bool assign_max(T& a, T b) {
if (a < b) {
a = b;
return true;
}
return false;
}
int main() {
ios_base::sync_with_stdio(false);
cin.tie(0);
cout.tie(0);
int n;
string s;
cin >> n >> s;
vector<ll> pr(1, 0), pg(1, 0), py(1, 0);
vector<ll> posr, posg, posy;
for (int i = 0; i < n; i++) {
pr.push_back(pr.back() + (s[i] == 'R'));
pg.push_back(pg.back() + (s[i] == 'G'));
py.push_back(py.back() + (s[i] == 'Y'));
if (s[i] == 'R') {
posr.push_back(i);
} else if (s[i] == 'G') {
posg.push_back(i);
} else {
posy.push_back(i);;
}
}
vector<vector<vector<vector<ll>>>> dp(py.back() + 1, vector<vector<vector<ll>>>(pr.back() + 1, vector<vector<ll>>(pg.back() + 1, vector<ll>(3, 2e9))));
dp[0][0][0][0] = 0;
dp[0][0][0][1] = 0;
dp[0][0][0][2] = 0;
for (int cy = 0; cy <= py.back(); cy++) {
for (int cr = 0; cr <= pr.back(); cr++) {
for (int cg = 0; cg <= pg.back(); cg++) {
for (ll lst = 0; lst < 3; lst++) {
if ((lst == 0 && cr == 0) || (lst == 1 && cg == 0) || (lst == 2 && cy == 0)) {
continue;
}
for (int olst = 0; olst < 3; olst++) {
if (olst == lst) {
continue;
}
ll add = 0;
if (lst == 0) {
add = max(0ll, pg[posr[cr - 1]] - cg) + max(0ll, py[posr[cr - 1]] - cy);
} else if (lst == 1) {
add = max(0ll, pr[posg[cg - 1]] - cr) + max(0ll, py[posg[cg - 1]] - cy);
} else if (lst == 2) {
add = max(0ll, pr[posy[cy - 1]] - cr) + max(0ll, pg[posy[cy - 1]] - cg);
}
assign_min(dp[cy][cr][cg][lst], dp[cy - (lst == 2)][cr - (lst == 0)][cg - (lst == 1)][olst] + add);
}
}
}
}
}
ll ans = min({dp[py.back()][pr.back()][pg.back()][0], dp[py.back()][pr.back()][pg.back()][1], dp[py.back()][pr.back()][pg.back()][2]});
cout << (ans == 2e9 ? -1 : ans) << "\n";
}
