#include <bits/stdc++.h>
using namespace std;
const int N=404;
char c;
int n,Cr,Cg,Cy,t,id,A,B,x,y;
int a[N];
int dp[N][N][4],dp_[N][N][4];
int R[N],G[N],Y[N];
vector < int > v[4];
main () {
cin>>n;
for (int i=1; i<=n; i++) {
cin>>c;
R[i]=R[i-1]+(c=='R');
G[i]=G[i-1]+(c=='G');
Y[i]=Y[i-1]+(c=='Y');
if (c=='R') a[i]=0;
else
if (c=='G') a[i]=1;
else
if (c=='Y') a[i]=2;
v[a[i]].push_back(i);
}
for (Cr=0; Cr<=v[0].size(); Cr++)
for (Cg=0; Cg<=v[1].size(); Cg++)
for (t=0; t<3; t++)
dp_[Cr][Cg][t]=1e9;
dp_[0][0][0]=0;
dp_[0][0][1]=0;
dp_[0][0][2]=0;
for (int i=1; i<=n; i++) {
for (Cr=0; Cr<=i && Cr<=v[0].size(); Cr++)
for (Cg=0; Cg+Cr<=i && Cg<=v[1].size(); Cg++)
for (t=0; t<3; t++) {
dp[Cr][Cg][t]=1e9;
}
for (Cr=0; Cr<=i && Cr<=v[0].size(); Cr++)
for (Cg=0; Cg+Cr<=i && Cg<=v[1].size(); Cg++) {
Cy=i-Cr-Cg;
if (Cy>v[2].size()) continue;
if (Cr) {
id=v[0][Cr-1];
A=max(0,Cg-G[id]),B=max(0,Cy-Y[id]);
dp[Cr][Cg][0]= min(dp_[Cr-1][Cg][1],dp_[Cr-1][Cg][2]) + A + B;
}
if (Cg) {
id=v[1][Cg-1];
A=max(0,Cr-R[id]),B=max(0,Cy-Y[id]);
dp[Cr][Cg][1]= min(dp_[Cr][Cg-1][0],dp_[Cr][Cg-1][2]) + A + B;
}
if (Cy) {
id=v[2][Cy-1];
A=max(0,Cr-R[id]),B=max(0,Cg-G[id]);
dp[Cr][Cg][2]= min(dp_[Cr][Cg][0],dp_[Cr][Cg][1]) + A + B;
}
}
for (Cr=0; Cr<=i && Cr<=v[0].size(); Cr++)
for (Cg=0; Cg+Cr<=i && Cg<=v[1].size(); Cg++)
for (t=0; t<=2; t++)
dp_[Cr][Cg][t]=dp[Cr][Cg][t];
}
int ans=1e9;
for (t=0; t<3; ++t) {
ans=min(ans,dp_[v[0].size()][v[1].size()][t]);
}
if (ans==1e9) ans=-1;
cout<<ans<<"\n";
}