#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, k, a, b;
  cin >> n >> k >> a >> b;
  string ans;
  int last = a > b;
  while (a != b && a && b) {
    if (b > a && !last) {
      ans += string(min(b - a, k), 'B');
      b -= min(b - a, k);
      last = 1;
    } else if (a > b && last) {
      ans += string(min(a - b, k), 'G');
      a -= min(a - b, k);
      last = 0;
    } else if (a < b && last) {
      ans += 'G';
      a--;
      last = 0;
    } else {
      ans += 'B';
      b--;
      last = 1;
    }
  }
  if (a && b && a == b) {
    while (a && b) {
      if (last)
        ans += "GB";
      else
        ans += "BG";
      a--;
      b--;
    }
    cout << ans << "\n";
  } else if (a == 0 && b == 0) {
    cout << ans << "\n";
  } else if (a) {
    if (a <= k) {
      ans += string(a, 'G');
    } else {
      cout << "NO\n";
      return 0;
    }
    cout << ans << "\n";
  } else if (b) {
    if (b <= k) {
      ans += string(b, 'B');
    } else {
      cout << "NO\n";
      return 0;
    }
    cout << ans << "\n";
  } else {
    cout << "NO\n";
  }
  return 0;
}
