#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k, g, b;
  cin >> n >> k >> g >> b;
  int maxx = max(g, b);
  int minn = min(g, b);
  char ma, mi;
  if (g >= b)
    ma = 'G', mi = 'B';
  else
    ma = 'B', mi = 'G';
  int i;
  string s;
  bool f = false;
  int maa = maxx;
  int mii = minn;
  if (minn == 0) {
    if (maxx <= k)
      for (i = 0; i < maxx; i++) cout << ma;
    else
      cout << "NO";
    return 0;
  }
  while (1) {
    if (f) {
      s += mi;
      minn--;
    } else {
      s += ma;
      maxx--;
    }
    f = !f;
    if (minn == 0) break;
  }
  if (maxx) {
    s += ma;
    maxx--;
  }
  int j;
  if (maxx) {
    int ff = maa - maxx;
    k--;
    if (ff * k >= maxx) {
      for (i = 0; i < s.length(); i++) {
        if (s[i] == ma && maxx > 0) {
          cout << s[i];
          if (maxx - k > 0) {
            for (j = 0; j < k; j++) cout << ma;
            maxx -= k;
          } else {
            for (j = 0; j < maxx; j++) cout << ma;
            maxx = 0;
          }
        } else
          cout << s[i];
      }
    } else
      cout << "NO";
    return 0;
  }
  cout << s;
  return 0;
}
