#include<bits/stdc++.h>
typedef int ll;
#define pii pair<ll, ll>
#define fi first
#define se second
#define endl '\n'
#define TASK ""
#define N 405
#define LOG 17
using namespace std;
const ll inf = 1e9;
bool ghuy4g;
ll n, pf[N][3], idx[N][3], sum[3];
string s;
map<char, ll> mp;
void pre() {
	mp['R'] = 0;
	mp['G'] = 1;
	mp['Y'] = 2;
	for (int i = 1; i <= n; i ++) {
		for (int z = 0; z < 3; z ++) {
			pf[i][z] = pf[i - 1][z];
		}
		pf[i][mp[s[i]]] ++ ;
		sum[mp[s[i]]] ++ ;
		idx[sum[mp[s[i]]]][mp[s[i]]] = i; // idx[i][z]: cai mau i thu z nam o vi tri nao
	}
}
int dp[402][402][402][3];
void solve() {
	for (int i = 0; i <= n; i ++) {
		for (int j = 0; j <= i; j ++) {
			for (int z = 0; z <= i; z ++) {
				for (int t = 0; t < 3; t ++) {
					dp[i][j][z][t] = inf;
				}
			}
		}
	}
	
	dp[0][0][0][0] = 0;
	dp[0][0][0][1] = 0;
	dp[0][0][0][2] = 0;
	
	for (int i = 1; i <= n; i ++) {
		for (int j = 0; j <= min(i, sum[0]); j ++) { // 0
			for (int z = 0; z <= min(i - j, sum[1]); z ++) { // 1
				dp[i][j][z][0] = inf;
				dp[i][j][z][1] = inf;
				dp[i][j][z][2] = inf;
				ll u = i - j - z; // 2
				if (u > sum[2]) continue;
				// dien 0
				if (j > 0) { // 0
					ll pos = idx[j][0]; // vi tri cua cai thu j mau 0
					ll them = 0;
					if (pf[pos][1] < z) {
						them += z - pf[pos][1];
					}
					if (pf[pos][2] < u) {
						them += u - pf[pos][2];
					}
					ll cost = abs(i - (pos + them));
					dp[i][j][z][0] = min(dp[i][j][z][0], dp[i - 1][j - 1][z][1] + cost);
					dp[i][j][z][0] = min(dp[i][j][z][0], dp[i - 1][j - 1][z][2] + cost);
				}
				if (z > 0) { // 1
					ll pos = idx[z][1];
					ll them = 0;
					if (pf[pos][0] < j) {
						them += j - pf[pos][0];
					}
					if (pf[pos][2] < u) {
						them += u - pf[pos][2];
					}
					ll cost = abs(i - (pos + them));
					dp[i][j][z][1] = min(dp[i][j][z][1], dp[i - 1][j][z - 1][0] + cost);
					dp[i][j][z][1] = min(dp[i][j][z][1], dp[i - 1][j][z - 1][2] + cost);
				}
				if (u > 0) { // 2
					ll pos = idx[u][2];
					ll them = 0;
					if (pf[pos][0] < j) {
						them += j - pf[pos][0];
					}
					if (pf[pos][1] < z) {
						them += z - pf[pos][1];
					}
					ll cost = abs(i - (pos + them));
					dp[i][j][z][2] = min(dp[i][j][z][2], dp[i - 1][j][z][0] + cost);
					dp[i][j][z][2] = min(dp[i][j][z][2], dp[i - 1][j][z][1] + cost);
				}
			}
		}
	}
	ll ans = min({dp[n][sum[0]][sum[1]][0], dp[n][sum[0]][sum[1]][1], dp[n][sum[0]][sum[1]][2]});
	if (ans >= inf) ans = -1;
	cout << ans;
}
bool klinh;
signed main() {
   // freopen("test.inp", "r", stdin);
	//freopen("test.out", "w", stdout);
	//srand(time(0));
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    
   	cin >> n;
   	cin >> s;
   	s = '*' + s;
   	
   	pre();
   	solve();
   	
   	cerr << fabs(&klinh - &ghuy4g) / double(1024 * 1024);
}
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