#include <bits/stdc++.h>
using namespace std;
#define ll short
#define ii pair<ll,ll>
#define iii tuple<ll,ll,ll>
const short maxn=205;
const short inf=10000;
short dp[maxn][maxn][maxn][5];
short cost[maxn][maxn][maxn][5];
vector<short> r,g,y;
short suff[405];
short solve(short a, short b, short c, short flag){
if (a<0 || b<0 || c<0) return inf;
if (a==0 && b==0 && c==0) return 0;
if (dp[a][b][c][flag]!=-1) return dp[a][b][c][flag];
if (flag==1){
dp[a][b][c][flag] = min(solve(a-1,b,c,2),solve(a-1,b,c,3));
}
if (flag==2){
dp[a][b][c][flag] = min(solve(a,b-1,c,1),solve(a,b-1,c,3));
}
if (flag==3){
dp[a][b][c][flag] = min(solve(a,b,c-1,1),solve(a,b,c-1,2));
}
dp[a][b][c][flag]+=cost[a][b][c][flag];
return dp[a][b][c][flag];
}
int main(){
ios_base::sync_with_stdio(0);
cin.tie(0);
short n;
cin>>n;
string s;
cin>>s;
for (short i=1;i<=n;i++){
if (s[i-1]=='R') r.emplace_back(i);
if (s[i-1]=='Y') y.emplace_back(i);
if (s[i-1]=='G') g.emplace_back(i);
}
if (2*max({r.size(),g.size(),y.size()})>n){
cout<<-1;
return 0;
}
for (int i=0;i<=r.size();i++){
for (int j=0;j<=g.size();j++){
//create suffix sum array
memset(suff,0,sizeof(suff));
for (int k=0;k<i;k++) suff[r[k]]=1;
for (int k=0;k<j;k++) suff[g[k]]=1;
for (int k=400;k>=0;k--) suff[k]+=suff[k+1];
for (int k=0;k<y.size();k++){
cost[i][j][k+1][3]=abs((suff[y[k]]+y[k])-(i+j+k+1));
}
}
}
for (int i=0;i<=g.size();i++){
for (int j=0;j<=y.size();j++){
//create suffix sum array
memset(suff,0,sizeof(suff));
for (int k=0;k<i;k++) suff[g[k]]=1;
for (int k=0;k<j;k++) suff[y[k]]=1;
for (int k=400;k>=0;k--) suff[k]+=suff[k+1];
for (int k=0;k<r.size();k++){
cost[k+1][i][j][1]=abs((suff[r[k]]+r[k])-(i+j+k+1));
}
}
}
for (int i=0;i<=y.size();i++){
for (int j=0;j<=r.size();j++){
//create suffix sum array
memset(suff,0,sizeof(suff));
for (int k=0;k<i;k++) suff[y[k]]=1;
for (int k=0;k<j;k++) suff[r[k]]=1;
for (int k=400;k>=0;k--) suff[k]+=suff[k+1];
for (int k=0;k<g.size();k++){
cost[j][k+1][i][2]=abs((suff[g[k]]+g[k])-(i+j+k+1));
}
}
}
memset(dp,-1,sizeof(dp));
cout<<min({solve(r.size(),g.size(),y.size(),1),
solve(r.size(),g.size(),y.size(),2),
solve(r.size(),g.size(),y.size(),3)});
return 0;
}
