#include <bits/stdc++.h>
using namespace std;
#define sz(x) (int)x.size()
int n;
char a[405];
int b[405];
vector <int> v[4];
int memo[405][405][405][4];
int dp(int x, int r, int g, int l) {
if (x == n + 1) return 0;
if (memo[x][r][g][l] != -1) return memo[x][r][g][l];
//cerr << x << ' ' << r << ' ' << g << ' ' << l << '\n';
int b = x - r - g - 1;
int ans = 1e9;
for (int i = 1; i <= 3; i++) {
if (i == l) continue;
if (i == 1 && r == sz(v[1])) continue;
if (i == 2 && g == sz(v[2])) continue;
if (i == 3 && b == sz(v[3])) continue;
int pos;
if (i == 1) {
pos = v[1][r];
int idx = upper_bound(v[2].begin(), v[2].end(), pos) - v[2].begin();
int idx2 = upper_bound(v[3].begin(), v[3].end(), pos) - v[3].begin();
//cerr << idx << ' ' << idx2 << '\n';
pos += max(0, g - idx) + max(0, b - idx2);
ans = min(ans, pos - x + dp(x + 1, r + 1, g, 1));
}
else if (i == 2) {
pos = v[2][g];
int idx = upper_bound(v[1].begin(), v[1].end(), pos) - v[1].begin();
int idx2 = upper_bound(v[3].begin(), v[3].end(), pos) - v[3].begin();
pos += max(0, r - idx) + max(0, b - idx2);
ans = min(ans, pos - x + dp(x + 1, r, g + 1, 2));
}
else {
pos = v[3][b];
int idx = upper_bound(v[2].begin(), v[2].end(), pos) - v[2].begin();
int idx2 = upper_bound(v[1].begin(), v[1].end(), pos) - v[1].begin();
pos += max(0, g - idx) + max(0, r - idx2);
ans = min(ans, pos - x + dp(x + 1, r, g, 3));
}
assert(pos>=x);
}
return memo[x][r][g][l] = ans;
}
int main(){
ios::sync_with_stdio(0);
cin.tie(0);
cin >> n;
for (int i = 1; i <= n; i++) {
cin >> a[i];
if (a[i] == 'R')b[i] = 1;
else if (a[i] == 'G') b[i] = 2;
else b[i] = 3;
v[b[i]].push_back(i);
}
memset(memo, -1, sizeof(memo));
int res = dp(1, 0, 0, 0);
cout << (res==1e9?-1:res);
}
