#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (n); i++)
typedef long long ll;
#define pb push_back
#define st first
#define nd second
const int MAXN = 401;
int INF = 1e9;
int dp[MAXN][MAXN][MAXN][3];
int jakie[MAXN][3][3];
int main() {
ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
int n;
cin >> n;
int T[n];
string s;
cin >> s;
vector<int> p[3];
rep(c, 3) {
p[c].pb(-1);
}
rep(i, n) {
if (s[i] == 'R') {
T[i] = 0;
}
else if (s[i] == 'G') {
T[i] = 1;
}
else {
T[i] = 2;
}
p[T[i]].pb(i);
}
int sz[3];
rep(c, 3) {
sz[c] = p[c].size();
}
if (n == 1) {
cout << 0 << '\n';
return 0;
}
int iter[3];
rep(c, 3) {
iter[c] = 0;
rep(d, 3) {
jakie[0][c][d] = 0;
}
}
rep(i, n) {
iter[T[i]]++;
rep(c, 3) {
jakie[iter[T[i]]][T[i]][c] = iter[c];
}
}
rep(i, n) {
rep(j, n) {
rep(k, n) {
rep(c, 3) {
dp[i][j][k][c] = INF;
}
}
}
}
rep(c, 3) {
dp[0][0][0][c] = 0;
}
rep(i, n) {
rep(j, n) {
rep(k, n) {
if ((i + j + k) == 0) {
continue;
}
if (i >= sz[0] || j >= sz[1] || k >= sz[2]) {
continue;
}
rep(c, 3) {
if (c == 0) {
if (i > 0) {
dp[i][j][k][c] = max(0, j - jakie[i][c][1]) + max(0, k - jakie[i][c][2]) + min(dp[i - 1][j][k][1], dp[i - 1][j][k][2]);
}
}
else if (c == 1) {
if (j > 0) {
dp[i][j][k][c] = max(0, i - jakie[j][c][0]) + max(0, k - jakie[j][c][2]) + min(dp[i][j - 1][k][0], dp[i][j - 1][k][2]);
}
}
else {
if (k > 0) {
dp[i][j][k][c] = max(0, i - jakie[k][c][0]) + max(0, j - jakie[k][c][1]) + min(dp[i][j][k - 1][0], dp[i][j][k - 1][1]);
}
}
// cout << "i = " << i << " j = " << j << " k = " << k << " c = " << c << " dp = " << dp[i][j][k][c] << '\n';
}
}
}
}
int ans = INF;
rep(c, 3) {
ans = min(ans, dp[sz[0] - 1][sz[1] - 1][sz[2] - 1][c]);
}
if (ans >= INF) {
cout << -1 << '\n';
return 0;
}
cout << ans << '\n';
return 0;
}
