#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 = 123456789;
const int N = 402;
int dp[N][N][N][4];
int lb[N][3];
int main() {
ios::sync_with_stdio(false);
cin.tie(0);
int n;
cin >> n;
string s;
cin >> s;
vector<vector<int>> at(3);
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 index_of_char, int type, int nR, int nG, int nY) {
if (type == 0) {
int bigger_gs = nG - lb[index_of_char][1];
int bigger_ys = nY - lb[index_of_char][2];
return index_of_char + max(0, bigger_gs) + max(0, bigger_ys);
} else if (type == 1) {
int bigger_rs = nR - lb[index_of_char][0];
int bigger_ys = nY - lb[index_of_char][2];
return index_of_char + max(0, bigger_rs) + max(0, bigger_ys);
} else {
int bigger_rs = nR - lb[index_of_char][0];
int bigger_gs = nG - lb[index_of_char][1];
return index_of_char + max(0, bigger_rs) + max(0, bigger_gs);
}
};
memset(dp, -1, sizeof(dp));
function<int(int, int, int, int)> DP = [&](int i, int nR, int nG, int previous_char) {
if (i == n) {
return 0;
}
if (dp[i][nR][nG][previous_char] != -1) {
return dp[i][nR][nG][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 dp[i][nR][nG][previous_char] = ans;
};
int res = DP(0, 0, 0, 3);
if (res == inf) {
cout << -1 << '\n';
} else {
cout << res << '\n';
}
return 0;
}
