#pragma GCC optimize("O3,unroll-loops")
#pragma GCC target("avx2,bmi,bmi2,lzcnt,popcnt")
#include <bits/stdc++.h>
using namespace std;
void solve() {
int n;
string s;
cin >> n >> s;
vector<int> pos[3];
for (int i = 0; i < n; i++) {
if (s[i] == 'R') {
pos[0].push_back(i);
} else if (s[i] == 'G') {
pos[1].push_back(i);
} else {
pos[2].push_back(i);
}
}
int R = pos[0].size(), G = pos[1].size(), Y = pos[2].size();
vector dp(R + 1, vector(G + 1, vector(Y + 1, vector<int>(4, 1e9))));
dp[0][0][0][3] = 0;
for (int i = 0; i < n; i++) {
for (int r = 0; r <= R && r <= i; r++) {
for (int g = 0; g <= G && r + g <= i; g++) {
int y = i - r - g;
if (y > Y) {
continue;
}
for (int l = 0; l < 4; l++) {
if (l != 0 && r < R) {
int a = lower_bound(pos[1].begin(), pos[1].end(), pos[0][r]) - pos[1].begin();
int b = lower_bound(pos[2].begin(), pos[2].end(), pos[0][r]) - pos[2].begin();
dp[r + 1][g][y][0] = min(dp[r + 1][g][y][0], dp[r][g][y][l] + max(0, g - a) + max(0, y - b));
}
if (l != 1 && g < G) {
int a = lower_bound(pos[0].begin(), pos[0].end(), pos[1][g]) - pos[0].begin();
int b = lower_bound(pos[2].begin(), pos[2].end(), pos[1][g]) - pos[2].begin();
dp[r][g + 1][y][1] = min(dp[r][g + 1][y][1], dp[r][g][y][l] + max(0, r - a) + max(0, y - b));
}
if (l != 2 && y < Y) {
int a = lower_bound(pos[0].begin(), pos[0].end(), pos[2][y]) - pos[0].begin();
int b = lower_bound(pos[1].begin(), pos[1].end(), pos[2][y]) - pos[1].begin();
dp[r][g][y + 1][2] = min(dp[r][g][y + 1][2], dp[r][g][y][l] + max(0, r - a) + max(0, g - b));
}
}
}
}
}
int ans = min({dp[R][G][Y][0], dp[R][G][Y][1], dp[R][G][Y][2]});
cout << (ans == 1e9 ? -1 : ans) << "\n";
}
int main() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
solve();
}
