#include <iostream>
#include <cstring>
using namespace std;
const int N = 405;
const int INF = N * N;
#define offset(i) (ro[rx][i] + go[gx][i] + yo[yx][i])
int n, rc = 0, gc = 0, yc = 0;
string s;
int ri[N], gi[N], yi[N];
int ro[N][N], go[N][N], yo[N][N];
int dp[N][N][N][3];
void precomp() {
for (int i = 0; i < n; i++) {
if (s[i] == 'R') {
ri[rc] = i;
rc++;
} else if (s[i] == 'G') {
gi[gc] = i;
gc++;
} else {
yi[yc] = i;
yc++;
}
for (int j = i - 1; j >= 0; j--) {
if (s[i] == 'R') ro[rc][j] = 1 + ro[rc - 1][j];
else if (s[i] == 'G') go[gc][j] = 1 + go[gc - 1][j];
else yo[yc][j] = 1 + yo[yc - 1][j];
}
}
for (int i = 0; i <= rc; i++) {
for (int j = 0; j <= gc; j++) {
for (int k = 0; k <= yc; k++) {
dp[i][j][k][0] = dp[i][j][k][1] = dp[i][j][k][2] = -1;
}
}
}
}
int solve(int rx, int gx, int yx, int last) {
if (rx == rc and gx == gc and yx == yc) return 0;
if (dp[rx][gx][yx][last] != -1) return dp[rx][gx][yx][last];
int &ret = dp[rx][gx][yx][last];
int len = rx + gx + yx;
int i = ri[rx];
ret = INF;
if (last != 0 and rx < rc) ret = min(ret, i + ro[rx][i] + go[gx][i] + yo[yx][i] - len + solve(rx + 1, gx, yx, 0));
i = gi[gx];
if (last != 1 and gx < gc) ret = min(ret, i + ro[rx][i] + go[gx][i] + yo[yx][i] - len + solve(rx, gx + 1, yx, 1));
i = yi[yx];
if (last != 2 and yx < yc) ret = min(ret, i + ro[rx][i] + go[gx][i] + yo[yx][i] - len + solve(rx, gx, yx + 1, 2));
return ret;
}
int main() {
ios_base::sync_with_stdio(false);
cin.tie(NULL);
cin >> n >> s;
precomp();
int ans = min(solve(0, 0, 0, 0), solve(0, 0, 0, 1));
cout << (ans >= INF ? -1 : ans) << '\n';
return 0;
}
