#include <bits/stdc++.h>
using namespace std;
int n, r, g, b;
char cc[210];
int main() {
  scanf("%d%s", &n, cc);
  for (int i = 0; i < n; i++)
    if (cc[i] == 'R')
      r++;
    else if (cc[i] == 'G')
      g++;
    else if (cc[i] == 'B')
      b++;
  if (r > 0 && g > 0 && b > 0) printf("BGR\n");
  if (r > 0 && g == 0 && b == 0) printf("R\n");
  if (r == 0 && g > 0 && b == 0) printf("G\n");
  if (r == 0 && g == 0 && b > 0) printf("B\n");
  if (r > 0 && g > 0 && b == 0) {
    if (r == 1 && g == 1) printf("B\n");
    if (r > 1 && g == 1) printf("BG\n");
    if (r == 1 && g > 1) printf("BR\n");
    if (r > 1 && g > 1) printf("BGR\n");
  }
  if (r > 0 && g == 0 && b > 0) {
    if (r == 1 && b == 1) printf("G\n");
    if (r > 1 && b == 1) printf("BG\n");
    if (r == 1 && b > 1) printf("GR\n");
    if (r > 1 && b > 1) printf("BGR\n");
  }
  if (r == 0 && g > 0 && b > 0) {
    if (b == 1 && g == 1) printf("R\n");
    if (b > 1 && g == 1) printf("GR\n");
    if (b == 1 && g > 1) printf("BR\n");
    if (b > 1 && g > 1) printf("BGR\n");
  }
  return 0;
}
