#include <bits/stdc++.h>
using namespace std;
void FastIO() {
  ios_base::sync_with_stdio(0);
  cin.tie(NULL);
  cout.tie(NULL);
}
int main() {
  FastIO();
  int n;
  cin >> n;
  string second;
  cin >> second;
  vector<int> m(256, 0);
  char r = 'R', g = 'G', b = 'B';
  for (int i = 0; i < second.length(); i++) {
    m[second[i]]++;
  }
  if (m[r] && m[g] && m[b]) {
    cout << "BGR" << '\n';
  } else if (m[r] && !m[g] && !m[b]) {
    cout << "R" << '\n';
  } else if (!m[r] && m[g] && !m[b]) {
    cout << "G" << '\n';
  } else if (!m[r] && !m[g] && m[b]) {
    cout << "B" << '\n';
  } else if (m[r] && m[g] && !m[b]) {
    second = "";
    if (m[r] > 1) second += "G";
    if (m[g] > 1) second += "R";
    second += "B";
    sort((second).begin(), (second).end());
    cout << second << '\n';
  } else if (m[r] && !m[g] && m[b]) {
    second = "";
    if (m[r] > 1) second += "B";
    if (m[b] > 1) second += "R";
    second += "G";
    sort((second).begin(), (second).end());
    cout << second << '\n';
  } else if (!m[r] && m[g] && m[b]) {
    second = "";
    if (m[g] > 1) second += "B";
    if (m[b] > 1) second += "G";
    second += "R";
    sort((second).begin(), (second).end());
    cout << second << '\n';
  }
  return 0;
}
