#include <bits/stdc++.h>
using namespace std;
#define forn(i,n) for(int i=0;i<int(n);i++)
#define forsn(i,s,n) for(int i=int(s);i<int(n);i++)
#define dforn(i,n) for(int i=int(n)-1;i>=0;i--)
#define dforsn(i,s,n) for(int i=int(n)-1;i>=int(s);i--)
#define fst first
#define snd second
#define pb push_back
#define eb emplace_back
#define sz(x) (int)x.size()
#define all(x) x.begin(), x.end()
typedef long long ll;
typedef vector<ll> vll;
typedef vector<int> vi;
typedef pair<int,int> ii;
const int N=405;
const int INF=1e9;
int dp[N][N][N][3];
void chmin(int &x, int v){
if(x>v) x=v;
}
int main(){
ios::sync_with_stdio(0);
cin.tie(0);
int n;
string s;
cin>>n>>s;
vi pos[3];
forn(i,n){
int t=s[i]=='R'?2:s[i]=='G';
pos[t].pb(i);
}
forn(i,N) forn(j,N) forn(k,N) forn(t,3) dp[i][j][k][t]=INF;
dp[0][0][0][0]=dp[0][0][0][1]=dp[0][0][0][2]=0;
forn(i,sz(pos[0])+1) forn(j,sz(pos[1])+1) forn(k,sz(pos[2])+1) forn(t,3){
if(i<sz(pos[0])&&t!=0) chmin(dp[i+1][j][k][0],dp[i][j][k][t]+abs(i+j+k-pos[0][i]));
if(j<sz(pos[1])&&t!=1) chmin(dp[i][j+1][k][1],dp[i][j][k][t]+abs(i+j+k-pos[1][j]));
if(k<sz(pos[2])&&t!=2) chmin(dp[i][j][k+1][2],dp[i][j][k][t]+abs(i+j+k-pos[2][k]));
}
int res=INF;
forn(t,3) chmin(res,dp[sz(pos[0])][sz(pos[1])][sz(pos[2])][t]);
if(res==INF) cout<<"-1\n";
else cout<<res/2<<'\n';
return 0;
}
