#include <bits/stdc++.h>
using namespace std;
const int inf = 1e9;
const long long INF = 1e18;
const int mod = 1e9 + 7;
int n;
char a[210];
bool cmp(int a, int b, int c) {
  if (a != 0 && b != 0) return true;
  if (a == 0 && b == 0) return c != 0;
  return a + b > 1 && c > 0;
}
int main() {
  cin >> n;
  scanf("%s", a);
  int R = 0, G = 0, B = 0;
  for (int i = 0; i < n; i++) switch (a[i]) {
      case 'R':
        R++;
        break;
      case 'G':
        G++;
        break;
      case 'B':
        B++;
        break;
    }
  if (cmp(R, G, B)) printf("B");
  if (cmp(R, B, G)) printf("G");
  if (cmp(G, B, R)) printf("R");
  cout << endl;
  return 0;
}
