#include <iostream>
using namespace std;
const int N = 405;
int R[N][N][N], B[N][N][N], G[N][N][N];
signed main(){
for (int i=0;i<N;i++){
for (int j=0;j<N;j++)
for (int k=(i + j == 0);k<N;k++)
R[i][j][k] = G[i][j][k] = B[i][j][k] = 1e9;
}
int n;
string s;
cin>>n>>s;
s = '0' + s;
for (int i=0;i<=n;i++){
if (s[i] == 'Y')
s[i] = 'B';
}
for (int i=1;i<=n;i++){
for (int j=n;j>=0;j--){
for (int k=n;k>=0;k--){
if (s[i] == 'B'){
if (k > 0)
R[i][j][k-1] = min(R[i][j][k-1], R[i-1][j][k]);
if (k > 0)
G[i][j][k-1] = min(G[i][j][k-1], G[i-1][j][k]);
B[i][0][k] = min(B[i][0][k], R[i-1][k][0] + k);
B[i][k][0] = min(B[i][k][0], G[i-1][k][0] + k);
B[i][j+1][k] = min(B[i][j+1][k], B[i-1][j][k] + j + k + 1);
B[i][j][k+1] = min(B[i][j][k+1], B[i-1][j][k] + j + k + 1);
}
if (s[i] == 'G'){
if (k > 0)
R[i][j-1][k] = min(R[i][j-1][k], R[i-1][j][k]);
if (k > 0)
B[i][j][k-1] = min(B[i][j][k-1], B[i-1][j][k]);
G[i][0][k] = min(G[i][0][k], R[i-1][0][k] + k);
G[i][k][0] = min(G[i][k][0], B[i-1][k][0] + k);
G[i][j+1][k] = min(G[i][j+1][k], G[i-1][j][k] + j + k + 1);
G[i][j][k+1] = min(G[i][j][k+1], G[i-1][j][k] + j + k + 1);
}
if (s[i] == 'R'){
if (j > 0)
B[i][j-1][k] = min(B[i][j-1][k], B[i-1][j][k]);
if (j > 0)
G[i][j-1][k] = min(G[i][j-1][k], G[i-1][j][k]);
R[i][0][k] = min(R[i][0][k], G[i-1][0][k] + k);
R[i][k][0] = min(R[i][k][0], B[i-1][0][k] + k);
R[i][j+1][k] = min(R[i][j+1][k], R[i-1][j][k] + j + k + 1);
R[i][j][k+1] = min(R[i][j][k+1], R[i-1][j][k] + j + k + 1);
}
}
}
}
int Mn = min(R[n][0][0], min(G[n][0][0], B[n][0][0]));
if (Mn == 1e9)
Mn = -1;
cout<<Mn<<'\n';
}
