#include <bits/stdc++.h>
using namespace std;
char s[] = "GB";
int main() {
  int n, k, a, b;
  scanf("%d%d%d%d", &n, &k, &a, &b);
  if (a < b) {
    swap(a, b);
    swap(s[0], s[1]);
  }
  int num = (a - 1) / k + 1;
  if (b < num - 1) {
    puts("NO");
    return 0;
  }
  while (num--) {
    int cnt = min(a, k);
    a -= cnt;
    while (cnt--) {
      putchar(s[0]);
    }
    cnt = 0;
    while (cnt < k && b > 0 && b > num - 1) {
      putchar(s[1]);
      ++cnt;
      --b;
    }
  }
  puts("");
  return 0;
}
