#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k, a, b;
  cin >> n >> k >> a >> b;
  bool impossible = false;
  string s;
  int curGroup = (a > b) ? 0 : 1;
  int groupSize = 0;
  for (int i = 0; i < n; ++i) {
    if (curGroup) {
      if (!b--) {
        impossible = true;
        break;
      }
      s.push_back('B');
    } else {
      if (!a--) {
        impossible = true;
        break;
      }
      s.push_back('G');
    }
    if (++groupSize == k || ((curGroup && a > b) || (!curGroup && b > a))) {
      curGroup = 1 - curGroup;
      groupSize = 0;
    }
  }
  cout << (impossible ? "NO" : s) << endl;
  return 0;
}
