#include <bits/stdc++.h>
#define L(i, j, k) for(int i = (j); i <= (k); i++)
#define R(i, j, k) for(int i = (j); i >= (k); i--)
#define ll long long
#define sz(a) ((int) a.size())
#define all(a) a.begin(), a.end()
#define vi vector<ll>
#define pb emplace_back
#define me(a, x) memset(a, x, sizeof(a))
#define fst first
#define snd second
#define ii pair<ll, ll>
using namespace std;
int n;
int main(){
ios::sync_with_stdio(false);
cin.tie(nullptr);
cin>>n;
string s;cin>>s;
vi g,r;
L(i,0,n-1){
if(s[i]=='G')g.pb(i);
else r.pb(i);
}
ll j=0,sum1=0,sum2=0;
for(int i=0;i<n;i+=2){
if(j<sz(g)){
sum1+=abs(g[j++]-i);
}else{
sum1=-1;
break;
}
}j=0;
for(int i=0;i<n;i+=2){
if(j<sz(r)){
sum2+=abs(r[j++]-i);
}else{
sum2=-1;
break;
}
}
cout<<max(sum1,sum2)<<endl;
}
