#include <bits/stdc++.h>
using namespace std;
long long MOD = 1000000007;
const int MX = 2e5 + 5;
const long long INF = 1e18;
const long double PI = 4 * atan((long double)1);
template <class T>
bool ckmin(T& a, const T& b) {
  return a > b ? a = b, 1 : 0;
}
template <class T>
bool ckmax(T& a, const T& b) {
  return a < b ? a = b, 1 : 0;
}
int num, num1, num2, num3, num4;
int test;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  ;
  cin >> num1 >> num2 >> num3 >> num4;
  string string;
  int count = 0;
  for (int i = 0; i < num1; i++) {
    if (num3 > num4) {
      if ((i >= 1 && string[i - 1] == 'B') || count != num2) {
        string += "G";
        num3--;
      } else {
        string += "B";
        num4--;
      }
    } else {
      if ((i >= 1 && string[i - 1] == 'G') || count != num2) {
        string += "B";
        num4--;
      } else {
        string += "G";
        num3--;
      }
    }
    if (i >= 1 && string[i - 1] == string[i]) {
      count++;
    } else {
      count = 1;
    }
  }
  if (num3 == 0 && num4 == 0) {
    cout << string << endl;
  } else {
    cout << "NO" << endl;
  }
}
