/**
____ ____ ____ ____ ____
||a |||t |||o |||d |||o ||
||__|||__|||__|||__|||__||
|/__\|/__\|/__\|/__\|/__\|
**/
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const int N_MAX = 400;
int N;
string S;
int A[N_MAX + 2][3], cnta;
int B[N_MAX + 2][3], cntb;
int C[N_MAX + 2][3], cntc;
int dp[N_MAX + 2][N_MAX + 2][N_MAX + 2][3];
void update (int &x, const int &y) {
if (y < x) {
x = y;
}
}
int main () {
ios_base::sync_with_stdio(false);
cin.tie(0);
cout.tie(0);
cin >> N;
cin >> S;
for (int i = 0; i < N; i++) {
if (S[i] == 'R') {
cnta++;
A[cnta][1] = cntb;
A[cnta][2] = cntc;
} else if (S[i] == 'G') {
cntb++;
B[cntb][2] = cntc;
B[cntb][0] = cnta;
} else {
cntc++;
C[cntc][0] = cnta;
C[cntc][1] = cntb;
}
}
for (int a = 0; a <= cnta; a++) {
for (int b = 0; b <= cntb; b++) {
for (int c = 0; c <= cntc; c++) {
dp[a][b][c][0] = INT_MAX / 2;
dp[a][b][c][1] = INT_MAX / 2;
dp[a][b][c][2] = INT_MAX / 2;
}
}
}
dp[0][0][0][0] = 0;
dp[0][0][0][1] = 0;
dp[0][0][0][2] = 0;
for (int a = 0; a <= cnta; a++) {
for (int b = 0; b <= cntb; b++) {
for (int c = 0; c <= cntc; c++) {
if (a < cnta) {
update(dp[a + 1][b][c][0], min(dp[a][b][c][2], dp[a][b][c][1]) + abs(b - A[a + 1][1]) + abs(c - A[a + 1][2]));
}
if (b < cntb) {
update(dp[a][b + 1][c][1], min(dp[a][b][c][0], dp[a][b][c][2]) + abs(c - B[b + 1][2]) + abs(a - B[b + 1][0]));
}
if (c < cntc) {
update(dp[a][b][c + 1][2], min(dp[a][b][c][1], dp[a][b][c][0]) + abs(a - C[c + 1][0]) + abs(b - C[c + 1][1]));
}
}
}
}
int answer = INT_MAX / 2;
answer = min(answer, dp[cnta][cntb][cntc][0]);
answer = min(answer, dp[cnta][cntb][cntc][1]);
answer = min(answer, dp[cnta][cntb][cntc][2]);
cout << (answer != INT_MAX / 2 ? answer / 2 : -1) << "\n";
return 0;
}
