#include <bits/stdc++.h>
using namespace std;
int main() {
  std::ios::sync_with_stdio(false);
  long long int n, k, a, b, i;
  char gg = 'G', bb = 'B';
  cin >> n >> k >> a >> b;
  if (a > b) {
    int t = a;
    a = b;
    b = t;
    char tt = gg;
    gg = bb;
    bb = tt;
  }
  if (max(a, b) > (min(a, b) + 1) * k) {
    cout << "NO\n";
    return 0;
  }
  if (a == b) {
    for (i = 0; i < n; i++) {
      if (i & 1)
        cout << gg;
      else
        cout << bb;
    }
  } else {
    int x = b / (a + 1), y = b % (a + 1);
    while (1) {
      for (i = 0; i < x; i++) {
        cout << bb;
        b--;
      }
      if (y) {
        cout << bb;
        y--;
        b--;
      }
      if (a) {
        cout << gg;
        a--;
      }
      if (a == 0 && b == 0) break;
    }
  }
  return 0;
}
