#include <bits/stdc++.h>
using namespace std;
int n, cnt0 = 3;
int g, r, b;
bool markb = true, markg = true, markr = true;
char id['Z']['Z'];
vector<char> st, m;
int tt[500];
int main() {
  scanf("%d\n", &n);
  for (int i = 1; i <= n; ++i) {
    char ch;
    scanf("%c", &ch);
    if (ch == 'B') {
      ++b;
      cnt0 -= markb;
      markb = false;
    }
    if (ch == 'G') {
      ++g;
      cnt0 -= markg;
      markg = false;
    }
    if (ch == 'R') {
      ++r;
      cnt0 -= markr;
      markr = false;
    }
  }
  if (b >= 1 && g >= 1 && r >= 1) {
    printf("BGR");
    return 0;
  }
  if (cnt0 == 2) {
    if (b) {
      printf("B");
      return 0;
    }
    if (g) {
      printf("G");
      return 0;
    }
    if (r) {
      printf("R");
      return 0;
    }
  }
  if (g == 0) {
    if (r >= 2 && b >= 2) {
      printf("BGR");
      return 0;
    }
  }
  if (r == 0) {
    if (g >= 2 && b >= 2) {
      printf("BGR");
      return 0;
    }
  }
  if (b == 0) {
    if (g >= 2 && r >= 2) {
      printf("BGR");
      return 0;
    }
  }
  markb = markg = markr = false;
  id['G']['G'] = 'G';
  id['R']['R'] = 'R';
  id['B']['B'] = 'B';
  id['G']['R'] = id['R']['G'] = 'B';
  id['B']['R'] = id['R']['B'] = 'G';
  id['G']['B'] = id['B']['G'] = 'R';
  if (b >= 1) m.push_back('B');
  if (b >= 2) m.push_back('B');
  if (g >= 1) m.push_back('G');
  if (g >= 2) m.push_back('G');
  if (r >= 1) m.push_back('R');
  if (r >= 2) m.push_back('R');
  for (int i = 1; i <= m.size(); ++i) tt[i] = i;
  do {
    st.clear();
    for (int i = 1; i <= m.size(); ++i) st.push_back(m[tt[i] - 1]);
    for (int i = 0; i < st.size() - 1;) {
      st.push_back(id[st[i]][st[i + 1]]);
      i += 2;
    }
    if (st.back() == 'B') markb = true;
    if (st.back() == 'G') markg = true;
    if (st.back() == 'R') markr = true;
  } while (next_permutation(tt + 1, tt + m.size() + 1));
  if (markb) printf("B");
  if (markg) printf("G");
  if (markr) printf("R");
  return 0;
  return 0;
}
