#include <bits/stdc++.h>
using namespace std;
int main() {
  long long int n, i, j, k, a, b;
  cin >> n >> k >> a >> b;
  if (a > b) {
    if (a > (b + 1) * k) {
      cout << "NO";
      return 0;
    }
    long long int c = 0;
    while (a > 0 && b > 0) {
      if (a > b) {
        if (c >= k) {
          cout << "B";
          c = 0;
          b--;
        } else {
          cout << "G";
          c++;
          a--;
        }
      } else {
        cout << "B";
        b--;
        c = 0;
      }
    }
    if (a > 0) {
      while (a--) cout << "G";
    }
    if (b > 0) {
      while (b--) cout << "B";
    }
  } else {
    if (b > (a + 1) * k) {
      cout << "NO";
      return 0;
    }
    long long int c = 0;
    while (a > 0 && b > 0) {
      if (b > a) {
        if (c >= k) {
          cout << "G";
          c = 0;
          a--;
        } else {
          cout << "B";
          c++;
          b--;
        }
      } else {
        cout << "G";
        a--;
        c = 0;
      }
    }
    if (a > 0) {
      while (a--) cout << "G";
    }
    if (b > 0) {
      while (b--) cout << "B";
    }
  }
  return 0;
}
