#include <bits/stdc++.h>
#define EmirBey ios_base::sync_with_stdio(NULL);cin.tie(0);cout.tie(0);
#define fopen(s) freopen(s".in","r",stdin);freopen(s".out","w",stdout)
#define all(x) (x).begin(), (x).end()
#define pb push_back
#define int long long
using namespace std;
const int N = 3e5+2;
const int INF = 1e9 + 1;
const int mod = 1e9+7;
int n, pref[3][N], cnt[3], ans = 1e18, dp[405][405][405][3];
vector<int> pos[3];
string s;
void solve() {
cin >> n >> s;
int x=0;
s='2'+s;
for (auto i : s) {
if (i == '2') continue;
x++;
for (int j = 0; j < 3; j++)pref[j][x] += pref[j][x - 1];
if (i == 'R') {
cnt[0]++;
pos[0].pb(x);
pref[0][x]++;
}else if(i == 'G') {
cnt[2]++;
pos[2].pb(x);
pref[2][x]++;
}else if(i == 'Y') {
cnt[1]++;
pos[1].pb(x);
pref[1][x]++;
}
}
for (int i = 0; i <= n; i++) {
for (int a = 0; a <= cnt[0]; a++) {
for (int b = 0; b <= cnt[1]; b++) {
for (int c = 0; c < 3; c++) dp[i][a][b][c] = 1e18;
}
}
}
dp[0][0][0][0] = dp[0][0][0][1] = dp[0][0][0][2] = 0;
for (int i = 1; i <= n; i++) {
for (int a = 0; a <= cnt[0]; a++) {
for (int b = 0; b <= cnt[1]; b++) {
if (a + b > i) continue;
int c = i - (a + b);
if (c > cnt[2]) continue;
for (int last = 0; last < 3; last++) {
if (last != 0 && a) {
int j = pos[0][a - 1];
int adj = max(0ll, b - pref[1][j]) + max(0ll, c - pref[2][j]);
int val = max(0ll, j + adj - i);
dp[i][a][b][0] = min(dp[i][a][b][0], dp[i - 1][a - 1][b][last] + val);
}if (last != 1 && b) {
int j = pos[1][b - 1];
int adj = max(0ll, a - pref[0][j]) + max(0ll, c - pref[2][j]);
int val = max(0ll, j + adj - i);
dp[i][a][b][1] = min(dp[i][a][b][1], dp[i - 1][a][b - 1][last] + val);
}if (last != 2 && c) {
int j = pos[2][c - 1];
int adj = max(0ll, b - pref[1][j]) + max(0ll, a - pref[0][j]);
int val = max(0ll, j + adj - i);
dp[i][a][b][2] = min(dp[i][a][b][2], dp[i - 1][a][b][last] + val);
}
}
if (i == n)ans = min({ans, dp[i][a][b][0], dp[i][a][b][1], dp[i][a][b][2]});
}
}
}
if (ans == 1e18) cout << "-1\n";
else cout << ans << '\n';
}
signed main() {
EmirBey;
int tt = 1;
while (tt--) {
solve();
}
}
