#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, i;
  map<char, int> mp;
  cin >> n;
  char a[205];
  cin >> a;
  for (i = 0; i <= n - 1; i++) mp[a[i]]++;
  if (mp['R'] >= 1 && mp['G'] >= 1 && mp['B'] >= 1)
    cout << "BGR";
  else if ((mp['R'] >= 1 && mp['G'] == 0 && mp['B'] == 0) ||
           (mp['R'] == 0 && mp['G'] == 1 && mp['B'] == 1))
    cout << "R";
  else if ((mp['R'] == 0 && mp['G'] >= 1 && mp['B'] == 0) ||
           (mp['R'] == 1 && mp['G'] == 0 && mp['B'] == 1))
    cout << "G";
  else if ((mp['R'] == 0 && mp['G'] == 0 && mp['B'] >= 1) ||
           (mp['R'] == 1 && mp['G'] == 1 && mp['B'] == 0))
    cout << "B";
  else if (mp['R'] == 1 && mp['G'] > 1 && mp['B'] == 0)
    cout << "BR";
  else if (mp['R'] == 1 && mp['G'] == 0 && mp['B'] > 1)
    cout << "GR";
  else if (mp['R'] == 0 && mp['G'] == 1 && mp['B'] > 1)
    cout << "GR";
  else if (mp['R'] > 1 && mp['G'] == 1 && mp['B'] == 0)
    cout << "BG";
  else if (mp['R'] > 1 && mp['G'] == 0 && mp['B'] == 1)
    cout << "BG";
  else if (mp['R'] == 0 && mp['G'] > 1 && mp['B'] == 1)
    cout << "BR";
  else
    cout << "BGR";
}
