#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);
  cout << setprecision(20);
  auto solve = [&]() -> string {
    long long int n, k, a, b;
    cin >> n >> k >> a >> b;
    long long int remG = a;
    long long int remB = b;
    string ans;
    vector<long long int> rem({remG, remB});
    vector<long long int> lab({'G', 'B'});
    long long int cur = (remG > remB) ? 0 : 1;
    while (true) {
      if (rem[cur] == 0) {
        return "NO";
      } else if (rem[cur] < rem[1 - cur]) {
        ans += lab[cur];
        rem[cur] -= 1;
      } else {
        long long int t = min(rem[cur], k);
        for (long long int i = 0; i < t; i++) ans += lab[cur];
        rem[cur] -= t;
      }
      if (rem[0] == 0 && rem[1] == 0) return ans;
      cur = 1 - cur;
    }
  };
  cout << solve() << endl;
  return 0;
}
