#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define ff first
#define ss second
#define all(x) (x).begin(), (x).end()
#define pb push_back
#define ertunt return
const int mod = 998244353;
#pragma GCC target ("avx2")
#pragma GCC optimization ("O3")
#pragma GCC optimization ("unroll-loops")
ll dist(string s,string t){
ll res=0;
for(ll i = 0; i < s.size(); i++){
for(ll j = i; j < t.size(); j++){
if(t[j]==s[i]){
for(ll k = j - 1; k >= i; k--){
res++;
swap(t[k],t[k+1]);
}
break;
}
}
}
return res;
}
int main() {
ios::sync_with_stdio(false);
cin.tie(0);
ll n;
cin >> n;
string s, t;
cin >> s;
ll r = 0,g = 0;
for(ll i = 0; i < n; i++){
if(s[i] == 'R')r++;
else g++;
}
string x = "", d = "";
if(abs(r-g) > 1){
cout << -1 ;
ertunt 0;
}
for(ll i = 0; i < min(r,g); i++){
x+="RG";
d+="GR";
}
if(r == g){
cout << min(dist(s,d),dist(s,x));
}
if(r > g) cout << dist(x,s);
if(r < g) cout << dist(d,s);
}
