#include <bits/stdc++.h>
using namespace std;
double gcd(double a, double b) { return a < 0.01 ? b : gcd(fmod(b, a), a); }
long long bpow(long long a, long long b, long long m) {
  a %= m;
  long long res = 1;
  while (b > 0) {
    if (b & 1) res = res * a % m;
    a = a * a % m;
    b >>= 1;
  }
  return res;
}
vector<int> parent, rnk;
void make_set(int v) {
  parent[v] = v;
  rnk[v] = 0;
}
int find_set(int v) {
  if (v == parent[v]) return v;
  return parent[v] = find_set(parent[v]);
}
void union_sets(int a, int b) {
  a = find_set(a);
  b = find_set(b);
  if (a != b) {
    if (rnk[a] < rnk[b]) swap(a, b);
    parent[b] = a;
    if (rnk[a] == rnk[b]) rnk[a]++;
  }
}
struct edge {
  int u, v, w;
  bool operator<(edge const& other) { return w < other.w; }
};
long long int n, k, y, h, m, t;
long long int kv, q, r, x, z, w, x1, x2, t1, t2, p, d, s;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  cin >> n;
  string s;
  cin >> s;
  for (long long i = 0; i < int(n); i++) {
    if (s[i] == 'R') x++;
    if (s[i] == 'G') y++;
    if (s[i] == 'B') z++;
  }
  if (x > 0 && y > 0 && z > 0)
    p = 1, q = 1, r = 1;
  else if (x == 0 && y > 0 && z > 0) {
    p = 1;
    if (y > 1) r = 1;
    if (z > 1) q = 1;
  } else if (y == 0 && x > 0 && z > 0) {
    q = 1;
    if (x > 1) r = 1;
    if (z > 1) p = 1;
  } else if (z == 0 && x != 0 && y != 0) {
    r = 1;
    if (y > 1) p = 1;
    if (x > 1) q = 1;
  } else if (x == 0 && y == 0 && z != 0) {
    r = 1;
  } else if (y == 0 && x != 0 && z == 0) {
    p = 1;
  } else {
    q = 1;
  }
  if (r) cout << "B";
  if (q) cout << "G";
  if (p) cout << "R";
  return 0;
}
