#include <bits/stdc++.h>
using namespace std;
const int N = 404;
void mini(int &X, int Y) {
if (X > Y) X = Y;
}
int n;
string s;
vector<int> pos[3] {{-1}, {-1}, {-1}}, num[3][N];
signed main(void) {
ios::sync_with_stdio(false); cin.tie(nullptr);
cin >> n >> s;
s = '&' + s;
for (int i = 1; i <= n; i++) {
if (s[i] == 'G') pos[0].emplace_back(i);
else if (s[i] == 'R') pos[1].emplace_back(i);
else pos[2].emplace_back(i);
}
int ng = pos[0].size() - 1, nr = pos[1].size() - 1, ny = pos[2].size() - 1;
vector<vector<vector<vector<int>>>> dp(ng + 3, vector<vector<vector<int>>>(nr + 3, vector<vector<int>>(ny + 3, vector<int>(3, N * N))));
for (int i = 0; i <= n; i++) {
num[0][i].resize(ng + 3, 0);
num[1][i].resize(nr + 3, 0);
num[2][i].resize(ny + 3, 0);
}
dp[0][0][0][0] = dp[0][0][0][1] = dp[0][0][0][2] = 0;
for (int i = 1; i <= n; i++) {
for (int x = 1; x <= ng; x++) {
num[0][i][x] = num[0][i][x - 1] + (pos[0][x] <= i);
}
for (int y = 1; y <= nr; y++) {
num[1][i][y] = num[1][i][y - 1] + (pos[1][y] <= i);
}
for (int z = 1; z <= ny; z++) {
num[2][i][z] = num[2][i][z - 1] + (pos[2][z] <= i);
}
}
auto shift = [&] (int x, int y, int z, int p) {
return x - num[0][p][x] + y - num[1][p][y] + z - num[2][p][z];
};
for (int x = 0; x <= ng; x++) {
for (int y = 0; y <= nr; y++) {
for (int z = 0; z <= ny; z++) {
int i = x + y + z;
if (x) {
mini(dp[x][y][z][0], min(dp[x - 1][y][z][1], dp[x - 1][y][z][2]) + pos[0][x] + shift(x, y, z, pos[0][x]) - i);
}
if (y) {
mini(dp[x][y][z][1], min(dp[x][y - 1][z][2], dp[x][y - 1][z][0]) + pos[1][y] + shift(x, y, z, pos[1][y]) - i);
}
if (z) {
mini(dp[x][y][z][2], min(dp[x][y][z - 1][0], dp[x][y][z - 1][1]) + pos[2][z] + shift(x, y, z, pos[2][z]) - i);
}
}
}
}
int res = min({dp[ng][nr][ny][0], dp[ng][nr][ny][1], dp[ng][nr][ny][2]});
cout << (res >= N * N ? -1 : res);
return 0;
}
