#include <bits/stdc++.h>
using namespace std;
char s[100005];
int main() {
  int n, k, a, b, i, j;
  while (scanf("%d%d%d%d", &n, &k, &a, &b) != EOF) {
    if (a == b) {
      for (i = 0; i < a; i++) printf("BG");
      printf("\n");
      continue;
    }
    int x = max(a, b);
    int y = min(a, b);
    if (k == 1) {
      if (x - y > 1)
        printf("NO\n");
      else {
        for (i = y; i > 0; i--) {
          if (a > b)
            printf("GB");
          else
            printf("BG");
        }
        if (a > b)
          printf("G");
        else
          printf("B");
        printf("\n");
      }
      continue;
    }
    memset(s, 0, sizeof(s));
    if (x - y * k > k)
      printf("NO");
    else {
      for (i = 0; i < k; i++) {
        if (a > b)
          s[i] = 'G';
        else
          s[i] = 'B';
      }
      if (a > b)
        s[k] = 'B';
      else
        s[k] = 'G';
      int kk = min(x / k, y);
      kk = min(kk, (x - y) / (k - 1));
      int xx = x - kk * k;
      int yy = y - kk;
      int zz = min(xx, yy);
      xx -= zz;
      yy -= zz;
      for (i = kk; i > 0; i--) printf("%s", s);
      for (i = zz; i > 0; i--) {
        if (a > b)
          printf("GB");
        else
          printf("BG");
      }
      for (i = xx; i > 0; i--) {
        if (a > b)
          printf("G");
        else
          printf("B");
      }
    }
    printf("\n");
  }
  return 0;
}
