#include <iostream>
using namespace std;
int dp[3][405][405][405]; //lst, cnt_r, cnt_g, idx
int pos[3][405], cnt[3], pre[3][405]; //RGY
int32_t main() {
int n; cin >> n;
string s; cin >> s; s = " " + s;
for (int i = 1; i <= n; ++i) {
for (int j = 0; j < 3; ++j) {
pre[j][i] = pre[j][i - 1];
if (s[i] == "RGY"[j])
++cnt[j], pos[j][cnt[j]] = i, pre[j][i]++;
}
}
for (int i = 0; i < 3; ++i)
dp[i][0][0][0] = 0; //base case
for (int cr = 0; cr <= cnt[0]; ++cr)
for (int cg = 0; cg <= cnt[1]; ++cg)
for (int i = 1; i <= n; ++i) {
int cy = i - cg - cr;
if ((cy < 0) || (cy > cnt[2])) continue;
if (cr > 0)
dp[0][cr][cg][i] = min(dp[1][cr - 1][cg][i - 1], dp[2][cr - 1][cg][i - 1])
+ cg - min(cg, pre[1][pos[0][cr]]) + cy - min(cy, pre[2][pos[0][cr]]);
else dp[0][cr][cg][i] = 1e9;
if (cg > 0)
dp[1][cr][cg][i] = min(dp[0][cr][cg - 1][i - 1], dp[2][cr][cg - 1][i - 1])
+ cr - min(cr, pre[0][pos[1][cg]]) + cy - min(cy, pre[2][pos[1][cg]]);
else dp[1][cr][cg][i] = 1e9;
if (cy > 0)
dp[2][cr][cg][i] = min(dp[1][cr][cg][i - 1], dp[0][cr][cg][i - 1])
+ cg - min(cg, pre[1][pos[2][cy]]) + cr - min(cr, pre[0][pos[2][cy]]);
else dp[2][cr][cg][i] = 1e9;
}
int ans = 1e9;
for (int i = 0; i < 3; ++i)
ans = min(ans, dp[i][cnt[0]][cnt[1]][n]);
cout << ((ans == 1e9) ? -1 : ans);
}
