#include <bits/stdc++.h>
using namespace std;
const int N = 1e3 + 10;
int main() {
  int n, m, l, r;
  cin >> n >> m >> l >> r;
  int vid = 0, kol = 0;
  string t = "";
  for (int i = 1; i <= n; i++) {
    if (kol == m) {
      if (vid == 0 && r == 0) {
        cout << "NO" << endl;
        return 0;
      } else if (vid == 1 && l == 0) {
        cout << "NO" << endl;
        return 0;
      } else if (vid == 0) {
        vid = 1;
        t += "B";
        r--;
        kol = 1;
      } else {
        vid = 0;
        t += "G";
        l--;
        kol = 1;
      }
    } else if (l > r) {
      if (vid == 0)
        kol++;
      else {
        vid = 0;
        kol = 1;
      }
      l--;
      t += "G";
    } else {
      if (vid == 1)
        kol++;
      else {
        vid = 1;
        kol = 1;
      }
      r--;
      t += "B";
    }
  }
  cout << t << endl;
}
