#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  string str;
  cin >> str;
  set<char> s;
  for (int i = 0; i < n; i++) {
    s.insert(str[i]);
  }
  string ans = "";
  for (set<char>::iterator it = s.begin(); it != s.end(); it++) {
    ans += (*it);
  }
  int l = ans.size();
  if (l == 3) {
    cout << ans;
  } else if (l == 2) {
    int a[3];
    fill(a, a + 3, 0);
    for (int i = 0; i < l; i++) {
      if (ans[i] == 'B') {
        a[0]++;
      } else if (ans[i] == 'G') {
        a[1]++;
      } else if (ans[i] == 'R') {
        a[2]++;
      }
    }
    string fi = "";
    if (a[0] == 0) {
      fi += "B";
    } else if (a[1] == 0) {
      fi += "G";
    } else if (a[2] == 0) {
      fi += "R";
    }
    int cnta = 0, cntb = 0;
    char chk = str[0], chkb;
    for (int i = 0; i < n; i++) {
      if (str[i] == chk) {
        cnta++;
      } else {
        chkb = str[i];
        cntb++;
      }
    }
    if (n > 2) {
      if (cnta >= 2) {
        fi += chkb;
      }
      if (cntb >= 2) {
        fi += chk;
      }
    }
    sort(fi.begin(), fi.end());
    cout << fi;
  } else {
    cout << ans;
  }
}
