#include <bits/stdc++.h>
using namespace std;
template <class A, class B>
inline bool mina(A &first, B second) {
  return (first > second) ? (first = second, 1) : 0;
}
template <class A, class B>
inline bool maxa(A &first, B second) {
  return (first < second) ? (first = second, 1) : 0;
}
map<char, int> cnt;
char c[] = {'B', 'G', 'R'};
int main() {
  int N;
  cin >> N;
  string s;
  cin >> s;
  for (int(i) = 0; (i) < (N); ++(i)) {
    cnt[s[i]]++;
  }
  if (cnt.size() == 3) {
    cout << "BGR" << endl;
    return 0;
  } else if (cnt.size() == 1) {
    cout << cnt.begin()->first << endl;
  } else {
    for (int(i) = 0; (i) < (3); ++(i)) {
      int cc = cnt[c[i]];
      if (cc == 0) {
        printf("%c", c[i]);
      } else {
        bool good = false;
        for (int(j) = 0; (j) < (3); ++(j)) {
          if (j != i && cnt[c[j]] >= 2) {
            good = true;
          }
        }
        if (good) printf("%c", c[i]);
      }
    }
  }
  return 0;
}
