#include <bits/stdc++.h>
#pragma GCC optimize("Ofast")
#pragma GCC target("avx,avx2,fma")
using namespace std;
#define int long long
#define nl '\n'
#define io ios_base::sync_with_stdio(false); cin.tie(0); cout.tie(0)
mt19937 rng((unsigned)chrono::steady_clock::now().time_since_epoch().count());
const int mod = 1000000007, mod2 = 998244353;
// change this
const int N = 405;
int n, si[3], arr[N], dp[N][N][N][3], pf[3][N];
vector<int> pos[3];
int sum(int i, int x, int y) {
// (x, y]
if (x > y) return 0;
return pf[i][y] - pf[i][x];
}
signed main() {
io;
cin >> n;
for (int i=0; i<3; i++) pos[i].push_back(0);
for (int i=1; i<=n; i++) {
char c;
cin >> c;
if (c == 'R') arr[i] = 0;
else if (c == 'G') arr[i] = 1;
else arr[i] = 2;
si[arr[i]]++; pf[arr[i]][i]++;
pos[arr[i]].push_back(i);
for (int j=0; j<3; j++) pf[j][i] += pf[j][i-1];
}
for (int i=0; i<=si[0]; i++) {
for (int j=0; j<=si[1]; j++) {
for (int k=0; k<=si[2]; k++) {
for (int tak=0; tak<3; tak++) {
if (!(i+j+k)) continue;
// i: R, j: G, k: Y, tak: which one to take
dp[i][j][k][tak] = 1e9;
for (int prev=0; prev<3; prev++) {
// what was the previous one?
if (prev == tak) continue; // collision
if (tak == 0) {
if (!i) break;
dp[i][j][k][tak] = min(dp[i][j][k][tak],
dp[i-1][j][k][prev]
+ sum(1, pos[1][j], pos[0][i])
+ sum(2, pos[2][k], pos[0][i])
);
} else if (tak == 1) {
if (!j) break;
dp[i][j][k][tak] = min(dp[i][j][k][tak],
dp[i][j-1][k][prev]
+ sum(0, pos[0][i], pos[1][j])
+ sum(2, pos[2][k], pos[1][j])
);
} else {
if (!k) break;
dp[i][j][k][tak] = min(dp[i][j][k][tak],
dp[i][j][k-1][prev]
+ sum(0, pos[0][i], pos[2][k])
+ sum(1, pos[1][j], pos[2][k])
);
}
}
}
}
}
}
int ans = 1e9;
for (int i=0; i<3; i++) ans = min(ans, dp[si[0]][si[1]][si[2]][i]);
cout << (ans >= 1e9 ? -1 : ans) << nl;
}
