#include <bits/stdc++.h>
using namespace std;
int main(int argc, char const* argv[]) {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  int n, k, a, b;
  cin >> n >> k >> a >> b;
  vector<int> sol;
  bool swapped = false;
  if (a < b) {
    swap(a, b);
    swapped = true;
  }
  if (a == b) {
    for (int i = 0; i < n / 2; i += 1) {
      sol.push_back(0);
      sol.push_back(1);
    }
  } else {
    int diff = a - b, last = -1;
    while (diff >= k) {
      if (b == 0) {
        if (a <= k) {
          for (int i = 0; i < k; ++i) sol.push_back(0);
          a -= k;
          diff = 0;
        } else {
          cout << "NO\n";
          return 0;
        }
      } else {
        a -= k;
        b -= 1;
        for (int i = 0; i < k; ++i) sol.push_back(0);
        sol.push_back(1);
        last = 1;
        diff -= (k - 1);
      }
    }
    if (a != 0) {
      a -= diff;
      for (int i = 0; i < diff; ++i) sol.push_back(0);
      while (a > 0) {
        sol.push_back(1);
        sol.push_back(0);
        a -= 1;
        b -= 1;
      }
    }
  }
  for (auto it : sol) {
    if (swapped) {
      if (it == 1) {
        cout << "G";
      } else {
        cout << "B";
      }
    } else {
      if (it == 1) {
        cout << "B";
      } else {
        cout << "G";
      }
    }
  }
  cout << "\n";
  return 0;
}
