#include <bits/stdc++.h>
using namespace std;
#define pii pair<int, int>
#define ff first
#define ss second
#define pb push_back
unordered_map<char, int> conv = {{'R', 0}, {'G', 1}, {'Y', 2}};
const int mxN = 402;
int dp[mxN][mxN][mxN][3];
int a[mxN];
vector<int> v[3];
int pref[3][mxN];
int suff[3][mxN];
int n;
int main() {
ios::sync_with_stdio(0); cin.tie(0);
cin >> n;
string temp; cin >> temp;
for (int i = 1; i <= n; i++) a[i] = conv[temp[i - 1]];
for (int i = 0; i < 3; i++) {
v[i] = {-1};
pref[i][0] = 0;
for (int j = 1; j <= n; j++) {
pref[i][j] = pref[i][j - 1];
if (a[j] == i) {
pref[i][j]++; v[i].pb(j);
}
}
suff[i][n + 1] = 0;
for (int j = n; j >= 1; j--) {
suff[i][j] = suff[i][j + 1];
if (a[j] == i) {
suff[i][j]++;
}
}
}
for (int i = 0; i <= n; i++) {
for (int j = 0; j <= n; j++) {
for (int k = 0; k <= n; k++) {
for (int l = 0; l < 3; l++) {
dp[i][j][k][l] = 1e8;
}
}
}
}
dp[0][0][0][0] = dp[0][0][0][1] = dp[0][0][0][2] = 0;
for (int a = 0; a <= pref[0][n]; a++) {
for (int b = 0; b <= pref[1][n]; b++) {
for (int c = 0; c <= pref[2][n]; c++) {
if (a + b + c == 0) continue;
if (a) {
dp[a][b][c][0] = min(dp[a - 1][b][c][1], dp[a - 1][b][c][2]) + max(b - pref[1][v[0][a]], 0) + max(c - pref[2][v[0][a]], 0);
}
if (b) {
dp[a][b][c][1] = min(dp[a][b - 1][c][0], dp[a][b - 1][c][2]) + max(a - pref[0][v[1][b]], 0) + max(c - pref[2][v[1][b]], 0);
}
if (c) {
dp[a][b][c][2] = min(dp[a][b][c - 1][0], dp[a][b][c - 1][1]) + max(a - pref[0][v[2][c]], 0) + max(b - pref[1][v[2][c]], 0);
}
// cerr << a << ' ' << b << ' ' << c << ":\n";
// for (int i = 0; i < 3; i++) cerr << dp[a][b][c][i] << ' ';
// cerr << '\n';
}
}
}
int ans = 1e8;
for (int i = 0; i < 3; i++) ans = min(ans, dp[pref[0][n]][pref[1][n]][pref[2][n]][i]);
cout << (ans == 1e8 ? -1 : ans) << '\n';
}
