#include <bits/stdc++.h>
using namespace std;
int p, q, r, s, t, u;
int a[25];
int b[25];
int dp[25][25];
int n;
int c[25];
int sgn(int x) {
  if (x > 0)
    return 1;
  else if (x < 0)
    return -1;
  else
    return 0;
}
int main() {
  int i, j;
  int n, k, a, b;
  cin >> n >> k >> a >> b;
  int p = max(a, b);
  int q = min(a, b);
  if ((long long int)p > (long long int)(q + 1) * k) {
    cout << "NO" << endl;
    return 0;
  }
  int cont = 0;
  char last = 'B';
  for (i = 1; i <= n; i++) {
    if (a > b) {
      if (last == 'B') cont = 0;
      if (cont < k) {
        cout << "G";
        cont++;
        last = 'G';
        a--;
      } else {
        cout << "B";
        cont = 1;
        last = 'B';
        b--;
      }
      continue;
    } else {
      if (last == 'G') cont = 0;
      if (cont < k) {
        last = 'B';
        cout << "B";
        cont++;
        b--;
      } else {
        cout << "G";
        cont = 1;
        last = 'G';
        a--;
      }
      continue;
    }
  }
  cout << endl;
  return 0;
}
