#include<bits/stdc++.h>
using namespace std;
const int maxn=405;
int inf=5e6;
int dp1[maxn][maxn][3],dp0[maxn][maxn][3];
int main(){
ios::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
int n;
string s;
cin>>n>>s;
vector<int>w1,w2,w0;
vector<int>ted0(n),ted1(n),ted2(n);
for(int i=0;i<n;i++){
if(i!=0){
ted1[i]=ted1[i-1];
ted0[i]=ted0[i-1];
ted2[i]=ted2[i-1];
}
if(s[i]=='Y'){
ted0[i]++;
w0.push_back(i);
continue;
}
if(s[i]=='G'){
ted1[i]++;
w1.push_back(i);
continue;
}
ted2[i]++;
w2.push_back(i);
}
for(int ind=0;ind<n;ind++){
for(int i=0;i<maxn;i++){
for(int j=0;j<maxn;j++){
for(int h=0;h<3;h++){
dp0[i][j][h]=dp1[i][j][h];
}
}
}
for(int i=0;i<maxn;i++){
for(int j=0;j<maxn;j++){
for(int h=0;h<3;h++){
int z=ind+1-i-j;
if(i+j>ind+1||(h==2&&i+j==ind+1)){
dp1[i][j][h]=inf;
continue;
}
if(h==0){
if(i<=0||i>(int)w0.size()){
dp1[i][j][h]=inf;
continue;
}
dp1[i][j][h]=min(dp0[i-1][j][1],dp0[i-1][j][2])+max(ted1[w0[i-1]]-j,0)+max(ted2[w0[i-1]]-z,0);
}
if(h==1){
if(j<=0||j>(int)w1.size()){
dp1[i][j][h]=inf;
continue;
}
dp1[i][j][h]=min(dp0[i][j-1][0],dp0[i][j-1][2])+max(ted0[w1[j-1]]-i,0)+max(ted2[w1[j-1]]-z,0);
//cout<<"ted: "<<ted[w1[j-1]]<<"\n";
}
if(h==2){
if(z<=0||z>(int)w2.size()){
dp1[i][j][h]=inf;
continue;
}
dp1[i][j][h]=min(dp0[i][j][0],dp0[i][j][1])+max(ted1[w2[z-1]]-j,0)+max(ted0[w2[z-1]]-i,0);
}
// cout<<ind<<" "<<i<<" "<<j<<" "<<h<<" "<<dp1[i][j][h]<<"\n";
}
}
}
}
int res=min(dp1[(int)w0.size()][(int)w1.size()][1],min(dp1[(int)w0.size()][(int)w1.size()][0],dp1[(int)w0.size()][(int)w1.size()][2]));
//cout<<dp1[(int)w0.size()][(int)w1.size()][0]<<" "<<dp1[(int)w0.size()][(int)w1.size()][1]<<" "<<dp1[(int)w0.size()][(int)w1.size()][2]<<"\n";
//cout<<dp0[(int)w0.size()][(int)w1.size()-1][0]<<" "<<dp0[(int)w0.size()][(int)w1.size()-1][2]<<"\n";
if(res>=inf){
cout<<-1<<"\n";
}
else{
cout<<res<<"\n";
}
}
