#include <iostream>
#include <iomanip>
#include <fstream>
#include <vector>
#include <array>
#include <algorithm>
#include <utility>
#include <map>
#include <queue>
#include <set>
#include <cmath>
#include <cstdio>
#include <cstring>
#define ll long long
#define ld long double
#define eps 1e-8
#define MOD 1000000007
#define INF 0x3f3f3f3f
#define INFLL 0x3f3f3f3f3f3f3f3f
// change if necessary
#define MAXN 410
using namespace std;
int n;
string s;
bool vis[MAXN][MAXN][MAXN][3];
int dp[MAXN][MAXN][MAXN][3];
int pref[MAXN][3];
vector<int> idx[3];
int ckmin(int &a, int b) {
return a = min(a, b);
}
// so far have r, g, y for each color
// and last char is last
int solve(int r, int g, int y, int last) {
if (r < 0 || g < 0 || y < 0) return INF;
if (vis[r][g][y][last]) return dp[r][g][y][last];
vis[r][g][y][last] = true;
dp[r][g][y][last] = INF;
if (r == 0 && g == 0 && y == 0) return dp[r][g][y][last] = 0;
int pr = r;
int pg = g;
int py = y;
if (last == 0) pr--;
else if (last == 1) pg--;
else py--;
if (pr < 0 || pg < 0 || py < 0) return dp[r][g][y][last];
int to_move = pr + pg + py;
if (last == 0) {
int cur = idx[0][r - 1];
to_move = cur - to_move;
to_move += max(0, g - pref[cur][1]);
to_move += max(0, y - pref[cur][2]);
} else if (last == 1) {
int cur = idx[1][g - 1];
to_move = cur - to_move;
to_move += max(0, r - pref[cur][0]);
to_move += max(0, y - pref[cur][2]);
} else {
int cur = idx[2][y - 1];
to_move = cur - to_move;
to_move += max(0, r - pref[cur][0]);
to_move += max(0, g - pref[cur][1]);
}
for (int i = 0; i < 3; i++) {
if (i == last) continue;
ckmin(dp[r][g][y][last], solve(pr, pg, py, i) + to_move);
}
return dp[r][g][y][last];
}
int main() {
/* cin.tie(0)->sync_with_stdio(0); */
cin >> n;
cin >> s;
for (int i = 0; i < n; i++) {
if (i) {
pref[i][0] = pref[i - 1][0];
pref[i][1] = pref[i - 1][1];
pref[i][2] = pref[i - 1][2];
}
switch (s[i]) {
case 'R':
pref[i][0]++;
idx[0].push_back(i);
break;
case 'G':
pref[i][1]++;
idx[1].push_back(i);
break;
case 'Y':
pref[i][2]++;
idx[2].push_back(i);
break;
}
}
int r = pref[n - 1][0];
int g = pref[n - 1][1];
int y = pref[n - 1][2];
int ans = min({solve(r, g, y, 0), solve(r, g, y, 1), solve(r, g, y, 2)});
if (ans == INF) {
cout << "-1\n";
} else {
cout << ans << '\n';
}
return 0;
}
