/*#pragma GCC optimize("O3")
#pragma GCC target ("avx2")
#pragma GCC optimize("Ofast")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
#pragma GCC optimize("unroll-loops")*/
#include <bits/stdc++.h>
#pragma GCC optimize ("unroll-loops,Ofast,O3")
#pragma GCC target("avx,avx2,fma")
//#define file(data) freopen(data".in", "r", stdin); freopen(data".out", "w", stdout);
#define pb push_back
//#define ios ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
#define all(data) data.begin() , data.end()
#define endl '\n'
//freopen("nenokku_easy.in", "r", stdin);
//freopen("nenokku_easy.out", "w", stdout);
#define int long long
#define pii pair < int, int >
#define pll pair < long long, long long >
using namespace std;
typedef long long ll;
const int N = 1e6 + 5;
const int M = 350;
const int mod = 1e9 + 7;
int n, a[N];
char s[N];
main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
//file("pieaters");
cin >> n;
int r = 0, g = 0;
for(int i = 1; i <= n; i++) {
cin >> s[i];
if(s[i] == 'R') ++r;
else g++;
}
if(g != r && g + 1 != r && g != r + 1) {
cout << -1;
} else {
int res1 = 1e9, res2 = 1e9;
if(g >= r) {
res1 = 0;
int num = 1;
for(int i = 1; i <= n; i++) {
if(s[i] == 'G') {
res1 += abs(num - i);
num += 2;
}
}
}
if(g <= r) {
res2 = 0;
int num = 1;
for(int i = 1; i <= n; i++) {
if(s[i] == 'R') {
res2 += abs(num - i);
num += 2;
}
}
}
cout << min(res1, res2);
}
}
/*
5 9 7
...##....
..#.##..#
..#....##
.##...#..
....#....
WS?EE??
*/
