#include <bits/stdc++.h>
using namespace std;
int dp[202][202][202];
int n;
string s;
int main() {
  ios::sync_with_stdio(false);
  cin >> n;
  cin >> s;
  int A = 0, B = 0, C = 0;
  for (int i = 0; i < n; i++)
    if (s[i] == 'R')
      A++;
    else if (s[i] == 'B')
      B++;
    else
      C++;
  dp[A + B + C][A][B] = 1;
  for (int sum = n; sum > 1; sum--)
    for (int i = 0; i <= sum; i++)
      for (int j = 0; j <= sum; j++)
        if (dp[sum][i][j]) {
          int k = sum - i - j;
          if (i > 0 && j > 0) dp[sum - 1][i - 1][j - 1] = 1;
          if (i > 0 && k > 0) dp[sum - 1][i - 1][j + 1] = 1;
          if (k > 0 && j > 0) dp[sum - 1][i + 1][j - 1] = 1;
          if (i > 1) dp[sum - 1][i - 1][j] = 1;
          if (j > 1) dp[sum - 1][i][j - 1] = 1;
          if (k > 1) dp[sum - 1][i][j] = 1;
        }
  string ans = "";
  if (dp[1][0][1]) ans += 'B';
  if (dp[1][0][0]) ans += 'G';
  if (dp[1][1][0]) ans += 'R';
  cout << ans << endl;
  return 0;
}
