#include <bits/stdc++.h>
using namespace std;
#define int long long
#define OYY LLONG_MAX
#define mod 998244353
#define faster ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL);
#define mid (start+end)/2
#define lim 65
#define fi first
#define se second
int n;// 1 2 3
int cev=LLONG_MAX;
vector<int> v[4];
int dp[lim][lim][lim][lim];
inline void dpp(int red,int gre,int yel,int sira,int renk,int tut,string st){
//cout<<red<<" "<<gre<<" "<<yel<<" "<<sira<<" "<<tut<<" "<<renk<<" "<<st+'Y'<<endl;
if(red==v[1].size() && gre==v[2].size() && yel==v[3].size()){
//return tut;
cev=min(cev,tut);
return ;
}
//if(~dp[sira][red][gre][yel])return dp[sira][red][gre][yel];
int yum=LLONG_MAX;
if(renk!=1 && red<v[1].size()){
//cout<<"r"<<red<<" "<<gre<<" "<<yel<<" "<<sira<<" "<<tut<<" "<<abs(v[1][red]-sira)<<" "<<st+'R'<<endl;
//yum=min(yum,dpp(red+1,gre,yel,sira+1,1,tut+abs(v[1][red]-sira),st+'R'));
dpp(red+1,gre,yel,sira+1,1,tut+abs(v[1][red]-sira),st+'R');
}
if(renk!=2 && gre<v[2].size()){
//cout<<"g"<<red<<" "<<gre<<" "<<yel<<" "<<sira<<" "<<tut<<" "<<abs(v[2][gre]-sira)<<" "<<st+'G'<<endl;
//yum=min(yum,dpp(red,gre+1,yel,sira+1,2,tut+abs(v[2][gre]-sira),st+'G'));
dpp(red,gre+1,yel,sira+1,2,tut+abs(v[2][gre]-sira),st+'G');
}
if(renk!=3 && yel<v[3].size()){
//cout<<"y"<<red<<" "<<gre<<" "<<yel<<" "<<sira<<" "<<tut<<" "<<abs(v[3][yel]-sira)<<" "<<st+'Y'<<endl;
//yum=min(yum,dpp(red,gre,yel+1,sira+1,3,tut+abs(v[3][yel]-sira),st+'Y'));
dpp(red,gre,yel+1,sira+1,3,tut+abs(v[3][yel]-sira),st+'Y');
}
//return dp[sira][red][gre][yel]=yum;
}
int32_t main(){
faster
memset(dp,-1,sizeof(dp));
cin>>n;
string s;cin>>s;
for(int i=0;i<n;i++){
if(s[i]=='R')v[1].push_back(i+1);
else if(s[i]=='G')v[2].push_back(i+1);
else v[3].push_back(i+1);
}
string h="";
//cout<<dpp(0,0,0,1,0,0,h)<<'\n';
dpp(0,0,0,1,0,0,h);
if(cev==LLONG_MAX){
cev=-2;
}
cout<<cev/2<<'\n';
return 0;
}
