#include <bits/stdc++.h>
using namespace std;
const int N = 510, mod = 1e8;
inline int read() {
  int x = 0, c = getchar(), f = 0;
  for (; c > '9' || c < '0'; f = c == '-', c = getchar())
    ;
  for (; c >= '0' && c <= '9'; c = getchar()) x = (x << 1) + (x << 3) + c - '0';
  return f ? -x : x;
}
void write(long long x) {
  if (x > 9) write(x / 10);
  putchar(x % 10 + '0');
}
int n, m, a, b, flag, blo, s, sheng, i, j;
int main() {
  n = read();
  m = read();
  a = read();
  b = read();
  if (a < b) swap(a, b), flag = 1;
  s = b + 1;
  blo = a / s;
  sheng = a % s;
  if (sheng) blo++;
  if (blo > m) {
    puts("NO");
    return 0;
  }
  if (sheng) blo--;
  for (i = 1; i < s; i++) {
    for (j = 1; j <= blo; j++)
      if (flag)
        putchar('B');
      else
        putchar('G');
    if (sheng) {
      sheng--;
      if (flag)
        putchar('B');
      else
        putchar('G');
    }
    if (flag)
      putchar('G');
    else
      putchar('B');
  }
  for (j = 1; j <= blo; j++)
    if (flag)
      putchar('B');
    else
      putchar('G');
  puts("");
  return 0;
}
