#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
template<class T> using o_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
int main() {
ll n;
cin >> n;
string s;
cin >> s;
ll r = 0, g = 0, y = 0;
multiset<ll> R;
multiset<ll> G;
multiset<ll> Y;
for(ll j = 0; j < n; j++){
char i = s[j];
if(i == 'R'){
r++;
R.insert(j);
}else if(i == 'G'){
g++;
G.insert(j);
}else if(i == 'Y'){
y++;
Y.insert(j);
}
}
if(max(max(r, g), y) > (n + 1)/2){
cout << -1 << endl;
return 0;
}
string s1 = "";
string s2 = "";
for(ll i = 0; i < n; i++){
if(i%2 == 0){
s1 += "R";
s2 += "G";
}else{
s1 += "G";
s2 += "R";
}
}
o_set<ll> o1;
o1.insert(999999);
ll ans = 1e17;
if(r == (n + 1)/2){
ll c1 = 0;
for(ll i = 0; i < n; i++){
if(i%2 == 0){
auto t = *R.begin();
R.erase(R.begin());
ll minus = o1.order_of_key(t);
c1 += t - minus;
o1.insert(t);
}else{
auto t = *G.begin();
G.erase(G.begin());
ll minus = o1.order_of_key(t);
c1 += t - minus;
o1.insert(t);
}
}
ans = min(ans, c1);
}
o_set<ll> o2;
o2.insert(999999);
if(g == (n + 1)/2){
for(ll j = 0; j < n; j++){
char i = s[j];
if(i == 'R'){
R.insert(j);
}else if(i == 'G'){
G.insert(j);
}else if(i == 'Y'){
Y.insert(j);
}
}
ll c1 = 0;
for(ll i = 0; i < n; i++){
if(i%2 == 1){
auto t = *R.begin();
R.erase(R.begin());
ll minus = o2.order_of_key(t);
c1 += t - minus;
o2.insert(t);
}else{
auto t = *G.begin();
G.erase(G.begin());
ll minus = o2.order_of_key(t);
c1 += t - minus;
o2.insert(t);
}
}
ans = min(ans, c1);
}
cout << ans << endl;
}
