/*
Editor: Abdelrahman Hossam
Nickname: Blobo2_Blobo2
IOI next year isA :)
*/
#pragma GCC optimize("Ofast,no-stack-protector,unroll-loops,fast-math,O3")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,sse4.2,popcnt,abm,mmx,avx2")
#pragma GCC optimize("-ffast-math")
#pragma GCC optimize("-funroll-loops")
#pragma GCC optimize("-funroll-all-loops,-fpeel-loops,-funswitch-loops")
#include<bits/stdc++.h>
using namespace std;
//#define int long long
#define endl "\n"
#define all(v) v.begin(),v.end()
#define gen(arr,n,nxt) generate(arr,arr+n,nxt)
#define Blobo2_el_7akim_elly_3ayz_yro7_IOI_w_3ayz_yakol_jilaty ios_base::sync_with_stdio(false);cin.tie(0);
#define EPS 0.00000001
const int mo=1e9+7,INF=1e18;
int nxt(){int x;cin>>x;return x;}
int dp[401][401][401][3],n;
vector<int>vy,vr,vg;
int tr[400],tg[400],ty[400];
int solve(int r=0,int g=0,int y=0,int last=0){
//cout<<r<<' '<<g<<' '<<y<<' '<<last<<endl;
int idx = r+g+y;
if(idx==n)return 0;
int &ret = dp[r][g][y][last];
if(ret != -1)return ret;
ret = 1e9;
if(idx == 0){
idx = vg[g];
if(g+1<=tg[n-1])
ret = min(ret,solve(r,g+1,y,1) + max(0,tr[idx] - r)+ max(0,ty[idx] - y) );
idx = vy[y];
if(y+1<=ty[n-1])
ret = min(ret,solve(r,g,y+1,2) + max(0,tr[idx] - r)+ max(0,tg[idx] - g) );
idx = vr[r];
if(r+1<=tr[n-1])
ret = min(ret,solve(r+1,g,y,0) + max(0,tg[idx] - g)+ max(0,ty[idx] - y) );
}
else if(last == 0){
idx = vg[g];
if(g+1<=tg[n-1])
ret = min(ret,solve(r,g+1,y,1) + max(0,tr[idx] - r)+ max(0,ty[idx] - y));
idx = vy[y];
if(y+1<=ty[n-1])
ret = min(ret,solve(r,g,y+1,2) + max(0,tr[idx] - r)+ max(0,tg[idx] - g) );
}
else if(last == 1){
idx = vr[r];
if(r+1<=tr[n-1])
ret = min(ret,solve(r+1,g,y,0) + max(0,tg[idx] - g)+ max(0,ty[idx] - y) );
idx = vy[y];
if(y+1<=ty[n-1])
ret = min(ret,solve(r,g,y+1,2) + max(0,tr[idx] - r)+ max(0,tg[idx] - g) );
}
else if(last == 2){
idx = vg[g];
if(g+1<=tg[n-1])
ret = min(ret,solve(r,g+1,y,1) + max(0,tr[idx] - r)+ max(0,ty[idx] - y) );
idx = vr[r];
if(r+1<=tr[n-1])
ret = min(ret,solve(r+1,g,y,0) + max(0,tg[idx] - g)+ max(0,ty[idx] - y) );
}
return ret;
}
signed main(){
Blobo2_el_7akim_elly_3ayz_yro7_IOI_w_3ayz_yakol_jilaty
n=nxt();
string s;
cin>>s;
int arr[3];
arr[0]=count(all(s),'R');
arr[1]=count(all(s),'Y');
arr[2]=count(all(s),'G');
sort(arr,arr+3);
int mn =0;
if(arr[0]>arr[1]+1)mn += arr[0] - (arr[1]+1);
if(arr[2]>arr[0]+arr[1]+1)mn+= arr[2] - (arr[0]+arr[1]+1);
if(mn){
cout<<-1<<endl;
return 0;
}
for(int i=0;i<n;i++){
if(s[i] == 'R')
vr.push_back(i),tr[i]++;
if(s[i] == 'G')
vg.push_back(i),tg[i]++;
if(s[i] == 'Y')
vy.push_back(i),ty[i]++;
if(!i)continue;
tr[i]+=tr[i-1];
tg[i]+=tg[i-1];
ty[i]+=ty[i-1];
}
memset(dp,-1,sizeof dp);
cout<<solve();
return 0;
}
