#include <bits/stdc++.h>
#define all(x) x.begin(), x.end()
using namespace std;
const int MAXN = 405;
const int INF = 1e9;
int dp[MAXN][MAXN][MAXN][3];
signed main() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
for (int i = 0; i < MAXN; ++i) {
for (int j = 0; j < MAXN; ++j) {
for (int k = 0; k < MAXN; ++k) {
for (int last = 0; last < 3; ++last) {
dp[i][j][k][last] = INF;
}
}
}
}
int n;
cin >> n;
string s;
cin >> s;
vector<int> a(n);
for (int i = 0; i < n; ++i) {
a[i] = (s[i] == 'R' ? 0 : (s[i] == 'G' ? 1 : 2));
}
vector<vector<int>> pref(3, vector<int>(n + 1));
array<vector<int>, 3> inds;
for (int i = 0; i < n; ++i) {
inds[a[i]].push_back(i);
pref[a[i]][i + 1] = 1;
pref[0][i + 1] += pref[0][i];
pref[1][i + 1] += pref[1][i];
pref[2][i + 1] += pref[2][i];
}
dp[0][0][0][0] = 0;
dp[0][0][0][1] = 0;
dp[0][0][0][2] = 0;
for (int r = 0; r <= pref[0].back(); ++r) {
for (int g = 0; g <= pref[1].back(); ++g) {
for (int y = 0; y <= pref[2].back(); ++y) {
for (int last = 0; last < 3; ++last) {
if (last == 0 && r > 0) {
int pos = inds[0][r - 1];
dp[r][g][y][last] = min(dp[r][g][y][last], min(dp[r - 1][g][y][1], dp[r - 1][g][y][2]) + max(0, pref[1][pos] - g) + max(0, pref[2][pos] - y));
}
if (last == 1 && g > 0) {
int pos = inds[1][g - 1];
dp[r][g][y][last] = min(dp[r][g][y][last], min(dp[r][g - 1][y][0], dp[r][g - 1][y][2]) + max(0, pref[0][pos] - r) + max(0, pref[2][pos] - y));
}
if (last == 2 && y > 0) {
int pos = inds[2][y - 1];
dp[r][g][y][last] = min(dp[r][g][y][last], min(dp[r][g][y - 1][0], dp[r][g][y - 1][1]) + max(0, pref[0][pos] - r) + max(0, pref[1][pos] - g));
}
}
}
}
}
int r = pref[0].back();
int g = pref[1].back();
int y = pref[2].back();
int ans = min(dp[r][g][y][0], min(dp[r][g][y][1], dp[r][g][y][2]));
cout << (ans == INF ? -1 : ans) << '\n';
}
