#include <bits/stdc++.h>
using namespace std;
vector<string> V;
int n, k, a, b;
int main() {
  scanf("%d %d %d %d", &n, &k, &a, &b);
  bool tka = a >= b;
  while (a || b) {
    if (tka && a == 0 || !tka && b == 0) {
      printf("NO\n");
      return 0;
    }
    if (tka)
      if (a >= b) {
        int tk = min(a, k);
        V.push_back(string(tk, 'G'));
        a -= tk;
      } else {
        V.push_back(string(1, 'G'));
        a--;
      }
    else if (b >= a) {
      int tk = min(b, k);
      V.push_back(string(tk, 'B'));
      b -= tk;
    } else {
      V.push_back(string(1, 'B'));
      b--;
    }
    tka = !tka;
  }
  for (int i = 0; i < V.size(); i++) printf("%s", V[i].c_str());
  printf("\n");
  return 0;
}
