#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=1e9; break; } }j=0; for(int i=0;i<n;i+=2){ if(j<sz(r)){ sum2+=abs(r[j++]-i); }else{ sum2=1e9; break; } } if(min(sum1,sum2)==-1)cout<<-1<<endl; else cout<<min(sum1,sum2)<<endl; }