#include <iostream>
using namespace std;
const int N = 405;
int R[2][N][N], B[2][N][N], G[2][N][N];
signed main(){
int n;
string s;
cin>>n>>s;
s = '0' + s;
for (int i : {0, 1}){
for (int j=0;j<=n;j++)
for (int k=0;k<=n;k++)
if (i + j + k > 0)
R[i][j][k] = G[i][j][k] = B[i][j][k] = 1e9;
}
for (int i=1;i<=n;i++){
for (int j=0;j<=n;j++){
if (s[i] == 'Y'){
B[1][0][j] = min(B[1][0][j], R[0][j][0] + j);
B[1][j][0] = min(B[1][j][0], G[0][j][0] + j);
B[1][0][j+1] = min(B[1][0][j+1], R[0][j][0] + j + 1);
B[1][j+1][0] = min(B[1][j+1][0], G[0][j][0] + j + 1);
}
if (s[i] == 'G'){
G[1][0][j] = min(G[1][0][j], R[0][0][j] + j);
G[1][j][0] = min(G[1][j][0], B[0][j][0] + j);
G[1][0][j+1] = min(G[1][0][j+1], R[0][0][j] + j + 1);
G[1][j+1][0] = min(G[1][j+1][0], B[0][j][0] + j + 1);
}
if (s[i] == 'R'){
R[1][0][j] = min(R[1][0][j], G[0][0][j] + j);
R[1][j][0] = min(R[1][j][0], B[0][0][j] + j);
R[1][0][j+1] = min(R[1][0][j+1], G[0][0][j] + j + 1);
R[1][j+1][0] = min(R[1][j+1][0], B[0][0][j] + j + 1);
}
for (int k=0;k<=n;k++){
int vl = j + k + 1;
if (s[i] == 'Y'){
if (k > 0 and R[0][j][k] < R[1][j][k-1])
R[1][j][k-1] = R[0][j][k];
if (k > 0 and G[0][j][k] < G[1][j][k-1])
G[1][j][k-1] = G[0][j][k];
if (B[0][j][k] + j + k + 1 < B[1][j+1][k])
B[1][j+1][k] = B[0][j][k] + vl;
if (B[0][j][k] + j + k + 1 < B[1][j][k+1])
B[1][j][k+1] = B[0][j][k] + vl;
}
if (s[i] == 'G'){
if (j > 0 and R[0][j][k] < R[1][j-1][k])
R[1][j-1][k] = R[0][j][k];
if (k > 0 and B[0][j][k] < B[1][j][k-1])
B[1][j][k-1] = B[0][j][k];
if (G[0][j][k] + j + k + 1 < G[1][j+1][k])
G[1][j+1][k] = G[0][j][k] + vl;
if (G[0][j][k] + j + k + 1 < G[1][j][k+1])
G[1][j][k+1] = G[0][j][k] + vl;
}
if (s[i] == 'R'){
if (j > 0 and B[0][j][k] < B[1][j-1][k])
B[1][j-1][k] = B[0][j][k];
if (j > 0 and G[0][j][k] < G[1][j-1][k])
G[1][j-1][k] = G[0][j][k];
if (R[0][j][k] + j + k + 1 < R[1][j+1][k])
R[1][j+1][k] = R[0][j][k] + vl;
if (R[0][j][k] + j + k + 1 < R[1][j][k+1])
R[1][j][k+1] = R[0][j][k] + vl;
}
}
}
for (int j=0;j<=n;j++){
for (int k=0;k<=n;k++){
R[0][j][k] = R[1][j][k];
G[0][j][k] = G[1][j][k];
B[0][j][k] = B[1][j][k];
G[1][j][k] = R[1][j][k] = B[1][j][k] = 1e9;
}
}
}
int Mn = min(R[0][0][0], min(G[0][0][0], B[0][0][0]));
if (Mn == 1e9)
Mn = -1;
cout<<Mn<<'\n';
}
