#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k, a, b;
  cin >> n >> k >> a >> b;
  string gg = "G", bb = "B";
  if (a < b) swap(a, b), swap(gg, bb);
  vector<string> va, vb;
  va.push_back(gg), a--;
  while (b > 0) {
    vb.push_back(bb);
    va.push_back(gg);
    a--, b--;
  }
  if (a < 0) va.erase(--va.end()), a++;
  while (a > 0 and va[0].size() < k) {
    for (int i = 0; i < va.size() and a > 0; i++, a--) va[i] += gg;
  }
  if (a)
    cout << "NO" << endl;
  else {
    for (int i = 0; i < vb.size(); i++) cout << va[i] << vb[i];
    if (va.size() > vb.size()) cout << va.back();
  }
  return 0;
}
