#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
using namespace std;
const int INF = 1e9;
int dp[401][401][401][3];
int pos[3][401];
int pre[3][3][401][401];
int main() {
int n;
string s;
cin >> n >> s;
vector<int> cnt(3, 0);
for (int i = 0; i < n; i++) {
int c = (s[i] == 'R' ? 0 : (s[i] == 'G' ? 1 : 2));
pos[c][++cnt[c]] = i + 1;
}
// Har bir rang uchun boshqa ranglarning nechtasi undan oldin kelishini hisoblash
for (int c1 = 0; c1 < 3; c1++) {
for (int c2 = 0; c2 < 3; c2++) {
if (c1 == c2) continue;
for (int i = 1; i <= cnt[c1]; i++) {
int cur_pos = pos[c1][i];
int c2_cnt = 0;
for (int j = 1; j <= cnt[c2]; j++) {
if (pos[c2][j] < cur_pos) c2_cnt++;
pre[c1][c2][i][j] = c2_cnt;
}
}
}
}
// DP jadvalini to'ldirish
for (int i = 0; i <= cnt[0]; i++)
for (int j = 0; j <= cnt[1]; j++)
for (int k = 0; k <= cnt[2]; k++)
for (int l = 0; l < 3; l++) dp[i][j][k][l] = INF;
if (cnt[0] > 0) dp[1][0][0][0] = pos[0][1] - 1;
if (cnt[1] > 0) dp[0][1][0][1] = pos[1][1] - 1;
if (cnt[2] > 0) dp[0][0][1][2] = pos[2][1] - 1;
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++) {
if (dp[r][g][y][last] == INF) continue;
for (int next = 0; next < 3; next++) {
if (next == last) continue;
int nr = r + (next == 0), ng = g + (next == 1), ny = y + (next == 2);
if (nr > cnt[0] || ng > cnt[1] || ny > cnt[2]) continue;
int p = pos[next][(next == 0 ? nr : (next == 1 ? ng : ny))];
int cost = (p - 1) + max(0, nr - pre[next][0][(next == 0 ? nr : (next == 1 ? ng : ny))][cnt[0]])
+ max(0, ng - pre[next][1][(next == 0 ? nr : (next == 1 ? ng : ny))][cnt[1]])
+ max(0, ny - pre[next][2][(next == 0 ? nr : (next == 1 ? ng : ny))][cnt[2]])
- (r + g + y);
// Soddaroq xarajat hisoblash:
int actual_cost = max(0, r - pre[next][0][(next==0?nr:(next==1?ng:ny))][cnt[0]]) +
max(0, g - pre[next][1][(next==0?nr:(next==1?ng:ny))][cnt[1]]) +
max(0, y - pre[next][2][(next==0?nr:(next==1?ng:ny))][cnt[2]]);
dp[nr][ng][ny][next] = min(dp[nr][ng][ny][next], dp[r][g][y][last] + (p - 1 + actual_cost) - (r+g+y));
}
}
}
}
}
int ans = min({dp[cnt[0]][cnt[1]][cnt[2]][0], dp[cnt[0]][cnt[1]][cnt[2]][1], dp[cnt[0]][cnt[1]][cnt[2]][2]});
cout << (ans >= INF ? -1 : ans) << endl;
return 0;
}
