#include <bits/stdc++.h>
#define fast cin.tie(0)->sync_with_stdio(0);
#define inf ((int)1e9)
using namespace std;
const int N = 403;
int dp[N][N][N][3];
int32_t main(){
fast
int n;
cin >> n;
string s;
cin >> s;
int cnt[3] = {0, 0, 0};
vector <vector<int> > pos(3);
for(int i = 0; i < n; i++) {
if(s[i] == 'R') {
cnt[0]++;
pos[0].push_back(i);
}
else if(s[i] == 'G') {
cnt[1]++;
pos[1].push_back(i);
}
else {
cnt[2]++;
pos[2].push_back(i);
}
}
pos[0].push_back(inf);
pos[1].push_back(inf);
pos[2].push_back(inf);
for(int r = 0; r <= cnt[0]; r++)
for(int g = 0; g <= cnt[1]; g++)
for(int y = 0; y <= cnt[2]; y++)
for(int last = 0; last < 3; last++)
dp[r][g][y][last] = inf;
dp[1][0][0][0] = pos[0][0];
dp[0][1][0][1] = pos[1][0];
dp[0][0][1][2] = pos[2][0];
for(int r = 0; r <= cnt[0]; r++) {
int ir = pos[0][r];
for(int g = 0; g <= cnt[1]; g++) {
int ig = pos[1][g];
for(int y = 0; y <= cnt[2]; y++) {
int iy = pos[2][y];
int ind = r + g + y;
for(int last : {0, 1, 2}) {
int val = dp[r][g][y][last];
if(val >= inf) continue;
//cout << r << " " << g << " " << y << "\n";
//cout << ir << " " << ig << " " << iy << " " << ind << "\n";
//cout << val << "\n";
if(last != 0) {
dp[r+1][g][y][0] = min(dp[r+1][g][y][0], val + abs(ir - ind));
}
if(last != 1) {
dp[r][g+1][y][1] = min(dp[r][g+1][y][1], val + abs(ig - ind));
}
if(last != 2) {
dp[r][g][y+1][2] = min(dp[r][g][y+1][2], val + abs(iy - ind));
}
}
}
}
}
int ans = inf;
for(int i = 0; i < 3; i++) {
ans = min(ans, dp[cnt[0]][cnt[1]][cnt[2]][i]);
}
if(ans == inf) cout << -1;
else cout << ans / 2;
}
