#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#include <bits/stdc++.h>
using namespace std;
const int MAXN = 405;
int dp[3][MAXN][MAXN][MAXN];
int antes[3][MAXN][MAXN];
vector<int> l[3];
string ORD = "RGY";
const int INF = 0x3f3f3f3f;
int32_t main() {
ios_base::sync_with_stdio(false);
cin.tie(0);
int n;
cin >> n;
vector<int> v(n + 1);
for (int i = 1; i <= n; i++) {
char c;
cin >> c;
for (int j = 0; j < 3; j++) {
if (c == ORD[j]) v[i] = j;
}
l[v[i]].push_back(i);
}
for (int i = 0; i < 3; i++) {
for (int j = 0; j < ((int)l[i].size()); j++) {
int p = 0;
for (int k = 1; k <= n; k++) {
antes[i][j][k] = antes[i][j][k - 1];
if ((p <= j) && (l[i][p] == k)) {
antes[i][j][k]++;
p++;
}
}
}
}
vector<int> t(3);
for (int i = 0; i < 3; i++) {
t[i] = ((int)l[i].size());
}
memset(dp, -1, sizeof dp);
dp[0][0][0][0] = dp[1][0][0][0] = dp[2][0][0][0] = 0;
for (int i = 1; i <= n; i++) {
for (int q1 = 0; q1 <= min(i, t[0]); q1++) {
for (int q2 = 0; q2 <= min(i - q1, t[1]); q2++) {
for (int c = 0; c < 3; c++) {
int q[3] = {q1, q2, i - q1 - q2};
bool flag = 0;
for (int x = 0; x < 3; x++) {
if (q[x] > t[x]) flag = 1;
}
if (flag) continue;
q[c]--;
if (q[c] < 0) continue;
int pegar = l[c][q[c]];
for (int ant = 0; ant < 3; ant++) {
if (ant == c) continue;
int val = dp[ant][i - 1][q[0]][q[1]];
if (val == -1) continue;
int cost = pegar - 1;
if (q[0] > 0) cost -= antes[0][q[0] - 1][pegar];
if (q[1] > 0) cost -= antes[1][q[1] - 1][pegar];
if (q[2] > 0) cost -= antes[2][q[2] - 1][pegar];
if (dp[c][i][q1][q2] == -1) dp[c][i][q1][q2] = (val + cost);
else dp[c][i][q1][q2] = min(dp[c][i][q1][q2], val + cost);
}
}
}
}
}
int ans = INF, a = l[0].size(), b = l[1].size();
for (int i = 0; i < 3; i++) {
if (dp[i][n][a][b] != -1) ans = min(ans, dp[i][n][a][b]);
}
if (ans == INF) cout << "-1\n";
else cout << ans << "\n";
}
