#include <bits/stdc++.h>
using namespace std;
int n, k, a, b;
string s;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cin >> n >> k >> a >> b;
  int lst = -1;
  while (a > b) {
    if (b == 0) {
      if (a <= k)
        for (int i = 1; i <= a; ++i) s += 'G';
      else {
        cout << "NO";
        return 0;
      }
      break;
    }
    int dd = min(k, a - b + 1);
    for (int i = 1; i <= dd; ++i) s += 'G';
    s += 'B';
    lst = 1;
    a -= dd;
    b--;
  }
  while (b > a) {
    if (a == 0) {
      if (b <= k)
        for (int i = 1; i <= b; ++i) s += 'B';
      else {
        cout << "NO";
        return 0;
      }
      break;
    }
    int dd = min(k, b - a + 1);
    for (int i = 1; i <= dd; ++i) s += 'B';
    s += 'G';
    lst = 0;
    b -= dd;
    a--;
  }
  if (a == 0 || b == 0)
    cout << s;
  else {
    while (a && b) {
      if (lst == 0)
        s += 'G', s += 'B', --a, --b;
      else
        s += 'B', s += 'G', --a, --b;
    }
    cout << s;
  }
  return 0;
}
