#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k, g, b;
  cin >> n >> k >> g >> b;
  bool swapped = false;
  if (g > b) {
    swap(g, b);
    swapped = true;
  }
  vector<int> ans;
  while (g) {
    ans.push_back(1);
    ans.push_back(1);
    g--;
    b--;
  }
  if (b != g) {
    ans.push_back(1);
    b--;
  }
  for (int i = 0; i < ans.size(); i++) {
    if (i % 2 == 0) {
      ans[i] = min(k, b + 1);
      b -= ans[i] - 1;
    }
  }
  int cons = 0;
  if (b == 0) {
    for (int i = 0; i < ans.size(); i++) {
      cons = 1;
      for (int j = 0; j < ans[i]; j++) {
        if (i % 2 == 1) {
          if (swapped)
            cout << "B";
          else
            cout << "G";
        } else {
          if (swapped)
            cout << "G";
          else
            cout << "B";
        }
        cons++;
      }
    }
    cout << endl;
  } else
    cout << "NO\n";
  return 0;
}
