#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
const int N = 400;
const int INF = 1000000000;
int ima[N+5][3];
int dp[N+5][N+5][N+5][3];
int tr[3];
int gde[3][N+5];
int main(){
ios_base::sync_with_stdio(false), cin.tie(0);
cout.precision(10);
cout << fixed;
int n;
cin >> n;
string s;
cin >> s;
for(auto &c : s){
if(c == 'R') c = '0';
else if(c == 'G') c = '1';
else c = '2';
}
for(int i=1; i<=n; i++){
for(int j=0; j<=2; j++) ima[i][j] = ima[i-1][j];
int k = s[i-1] - '0';
ima[i][k]++;
++tr[k];
gde[k][tr[k]] = i;
}
for(int i=0; i<=ima[n][0]; i++) for(int j=0; j<=ima[n][1]; j++) for(int k=0; k<=ima[n][2]; k++) for(int l=0; l<=2; l++) dp[i][j][k][l] = INF;
for(int i=0; i<=2; i++) dp[0][0][0][i] = 0;
for(int i=0; i<=ima[n][0]; i++){
for(int j=0; j<=ima[n][1]; j++){
for(int k=0; k<=ima[n][2]; k++){
for(int l=0; l<=2; l++){
if(i + j + k > 0){
if(i == 0 && l == 0) continue;
if(j == 0 && l == 1) continue;
if(k == 0 && l == 2) continue;
}
if(i < ima[n][0] && l != 0){
int h = gde[0][i+1];
dp[i+1][j][k][0] = min(dp[i+1][j][k][0], dp[i][j][k][l] + max(0, j - ima[h][1]) + max(0, k - ima[h][2]));
}
if(j < ima[n][1] && l != 1){
int h = gde[1][j+1];
dp[i][j+1][k][1] = min(dp[i][j+1][k][1], dp[i][j][k][l] + max(0, i - ima[h][0]) + max(0, k - ima[h][2]));
}
if(k < ima[n][2] && l != 2){
int h = gde[2][k+1];
dp[i][j][k+1][2] = min(dp[i][j][k+1][2], dp[i][j][k][l] + max(0, i - ima[h][0]) + max(0, j - ima[h][1]));
}
}
}
}
}
int g = INF;
for(int i=0; i<=2; i++) g = min(g, dp[ima[n][0]][ima[n][1]][ima[n][2]][i]);
cout << (g == INF ? -1 : g) << "\n";
return 0;
}
