#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;
typedef pair<ll,ll> pl;
typedef pair<int,int> pii;
typedef tuple<int,int,int> tt;
#define all(a) a.begin(), a.end()
#define filter(a) a.erase(unique(all(a)), a.end())
int dp[3][404][404][404];
int pr[404], pg[404], py[404];
int main()
{
ios::sync_with_stdio(0);
cin.tie(0);
int n, cr = 0, cg = 0, cy = 0; cin >> n;
for (int i = 1; i <= n; i++) {
char c; cin >> c;
if (c == 'R') pr[++cr] = i;
else if (c == 'G') pg[++cg] = i;
else py[++cy] = i;
}
/*if (cg < cy)
swap(cg, cy), swap(pg, py);
if (cr < cg)
swap(cr, cg), swap(pr, pg);*/
for (int last = 0; last < 3; last++)
for (int i = 0; i <= cr; i++)
for (int j = 0; j <= cg; j++)
for (int k = 0; k <= cy; k++)
dp[last][i][j][k] = INT_MAX;
dp[0][0][0][0] = dp[1][0][0][0] = dp[2][0][0][0] = 0;
function<void(int,int,int,int)> refine = [&] (int i, int j, int k, int last) {
int cur = i + j + k;
if (last == 0) {
if (i > 0 && dp[1][i - 1][j][k] != INT_MAX)
dp[0][i][j][k] = min(dp[0][i][j][k], dp[1][i - 1][j][k] + abs(cur - pr[i]));
if (i > 0 && dp[2][i - 1][j][k] != INT_MAX)
dp[0][i][j][k] = min(dp[0][i][j][k], dp[2][i - 1][j][k] + abs(cur - pr[i]));
}
else if (last == 1) {
if (j > 0 && dp[0][i][j - 1][k] != INT_MAX)
dp[1][i][j][k] = min(dp[1][i][j][k], dp[0][i][j - 1][k] + abs(cur - pg[j]));
if (j > 0 && dp[2][i][j - 1][k] != INT_MAX)
dp[1][i][j][k] = min(dp[1][i][j][k], dp[2][i][j - 1][k] + abs(cur - pg[j]));
}
else if (last == 2) {
if (k > 0 && dp[0][i][j][k - 1] != INT_MAX)
dp[2][i][j][k] = min(dp[2][i][j][k], dp[0][i][j][k - 1] + abs(cur - py[k]));
if (k > 0 && dp[1][i][j][k - 1] != INT_MAX)
dp[2][i][j][k] = min(dp[2][i][j][k], dp[1][i][j][k - 1] + abs(cur - py[k]));
}
};
for (int i = 0; i <= cr; i++) {
for (int j = 0; j <= cg; j++) {
for (int k = 0; k <= cy; k++) {
if (!i && !j && !k) continue;
for (int last = 0; last < 3; last++)
refine(i, j, k, last);
}
}
}
int opt = min({dp[0][cr][cg][cy], dp[1][cr][cg][cy], dp[2][cr][cg][cy]});
cout << (opt == INT_MAX ? -1 : opt / 2);
return 0;
}
