#include <bits/stdc++.h>
using namespace std;
int n, k, a, b;
int main() {
  cin >> n >> k >> a >> b;
  int ta = (a) / k;
  int tb = (b) / k;
  if (a < (b - 1) / k || b < (a - 1) / k) {
    printf("NO\n");
    return 0;
  }
  int vis = 0;
  if (ta < tb) vis = 1;
  int t = max(ta, tb);
  int lena = k;
  int lenb = k;
  if (t) lena = max(1, min(lena, a / t)), lenb = max(1, min(lenb, b / t));
  for (int i = 1; i <= t; i++) {
    if (!vis) {
      for (int j = 1; j <= min(lena, a); j++) printf("G");
      a -= lena;
      for (int j = 1; j <= min(lenb, b); j++) printf("B");
      b -= lenb;
      while (a - lena * (t - i) > k && a - (lena + 1) * (t - i) >= 0 &&
             lena < k)
        lena++;
      while ((b - lenb * (t - i) > k ||
              a - lena * (t - i) == 0 && lenb + b - lenb * (t - i) > k) &&
             b - (lenb + 1) * (t - i) >= 0 && lenb < k)
        lenb++;
    } else {
      for (int j = 1; j <= min(lenb, b); j++) printf("B");
      b -= lenb;
      for (int j = 1; j <= min(lena, a); j++) printf("G");
      a -= lena;
      while ((a - lena * (t - i) > k ||
              b - lenb * (t - i) == 0 && lena + a - lena * (t - i) > k) &&
             a - (lena + 1) * (t - i) >= 0 && lena < k)
        lena++;
      while (b - lenb * (t - i) > k && b - (lenb + 1) * (t - i + 1) >= 0 &&
             lenb < k)
        lenb++;
    }
  }
  if (!vis) {
    for (int j = 1; j <= max(0, a); j++) printf("G");
    for (int j = 1; j <= max(0, b); j++) printf("B");
  } else {
    for (int j = 1; j <= max(0, b); j++) printf("B");
    for (int j = 1; j <= max(0, a); j++) printf("G");
  }
  return 0;
}
