#include <bits/stdc++.h>
using namespace std;
#ifdef LOCAL
#include "C:\GCC\debug.h"
#else
#define debug(...) void(42)
#endif
int Convert(char c) {
if (c == 'R') {
return 0;
}
if (c == 'G') {
return 1;
}
return 2;
}
const int inf = 166789;
const int N = 402;
int dp[N][N][N][3];
int lb[N][3];
vector<int> at[3];
int main() {
ios::sync_with_stdio(false);
cin.tie(0);
int n;
cin >> n;
string s;
cin >> s;
vector<int> cnt(26);
for (int i = 0; i < n; i++) {
cnt[s[i] - 'A'] += 1;
}
vector<int> order(26);
iota(order.begin(), order.end(), 0);
sort(order.begin(), order.end(), [&](int i, int j) {
return cnt[i] > cnt[j];
});
vector<char> trans(26);
trans[order[0]] = 'Y';
trans[order[1]] = 'G';
trans[order[2]] = 'R';
for (int i = 0; i < n; i++) {
s[i] = trans[s[i] - 'A'];
}
for (int i = 0; i < n; i++) {
at[Convert(s[i])].push_back(i);
}
lb[0][0] = lb[0][1] = lb[0][2] = 0;
lb[0][Convert(s[0])] = 1;
for (int i = 1; i < n; i++) {
lb[i][0] = lb[i - 1][0];
lb[i][1] = lb[i - 1][1];
lb[i][2] = lb[i - 1][2];
lb[i][Convert(s[i])] += 1;
}
auto Advance = [&](int loc, int type, int nR, int nG, int nY) {
int bRs = max(0, nR - lb[loc][0]);
int bGs = max(0, nG - lb[loc][1]);
int bYs = max(0, nY - lb[loc][2]);
if (type == 0) {
return loc + max(0, bGs) + max(0, bYs);
} else if (type == 1) {
return loc + max(0, bRs) + max(0, bYs);
}
return loc + max(0, bRs) + max(0, bGs);
};
for (int i = 0; i <= (int) at[0].size(); i++) {
for (int j = 0; j <= (int) at[1].size(); j++) {
for (int p = 0; p <= 2; p++) {
dp[n][i][j][p] = 0;
}
}
}
for (int i = n - 1; i >= 0; i--) {
for (int nR = min((int) at[0].size(), i); nR >= 0; nR--) {
for (int nG = min(i - nR, (int) at[1].size()); nG >= 0; nG--) {
dp[i][nR][nG][0] = dp[i][nR][nG][1] = dp[i][nR][nG][2] = inf;
auto Compute = [&](int previous_char) {
int nY = i - (nR + nG);
int ans = inf;
if (nR < (int) at[0].size() && previous_char != 0) {
ans = min(ans, dp[i + 1][nR + 1][nG][0] + max(0, Advance(at[0][nR], 0, nR, nG, nY) - i));
}
if (nG < (int) at[1].size() && previous_char != 1) {
ans = min(ans, dp[i + 1][nR][nG + 1][1] + max(0, Advance(at[1][nG], 1, nR, nG, nY) - i));
}
if (nY < (int) at[2].size() && previous_char != 2) {
ans = min(ans, dp[i + 1][nR][nG][2] + max(0, Advance(at[2][nY], 2, nR, nG, nY) - i));
}
return ans;
};
if (i > 0) {
for (int previous_char = 0; previous_char <= 2; previous_char++) {
dp[i][nR][nG][previous_char] = min(dp[i][nR][nG][previous_char], Compute(previous_char));
}
} else {
dp[i][nR][nG][0] = min(dp[i][nR][nG][0], Compute(-1));
}
}
}
}
if (dp[0][0][0][0] == inf) {
cout << -1 << '\n';
} else {
cout << dp[0][0][0][0] << '\n';
}
return 0;
}
