#include <bits/stdc++.h>
using namespace std;
struct TEA {
  int cnt;
  char tea;
} A, B;
int n, k;
void Pr(char ch, int cnt) {
  for (int i = 1; i <= cnt; i++) printf("%c", ch);
}
int main() {
  scanf("%d%d%d%d", &n, &k, &A.cnt, &B.cnt);
  A.tea = 'G';
  B.tea = 'B';
  if (A.cnt < B.cnt) swap(A, B);
  if (A.cnt > (long long)(B.cnt + 1) * k)
    printf("NO\n");
  else {
    while (true) {
      if (B.cnt == 0) break;
      if (A.cnt - k < B.cnt - 1) {
        Pr(A.tea, A.cnt - B.cnt);
        A.cnt = B.cnt;
        break;
      } else {
        Pr(A.tea, k);
        Pr(B.tea, 1);
        A.cnt -= k;
        B.cnt -= 1;
      }
    }
    if (B.cnt != 0) {
      while (A.cnt != 0) {
        Pr(B.tea, 1);
        Pr(A.tea, 1);
        A.cnt--;
        B.cnt--;
      }
    } else
      Pr(A.tea, A.cnt);
    printf("\n");
  }
}
