#include <bits/stdc++.h>
using namespace std;
template <typename T>
void out(T x) {
  cout << x << endl;
  exit(0);
}
const long long mod = 1e9 + 7;
const int maxn = 1e6 + 5;
int n, k, a, b;
int ans[maxn];
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  cin >> n >> k >> a >> b;
  map<int, int> mp;
  mp[0] = a;
  mp[1] = b;
  int y = (mp[0] > mp[1] ? 1 : 0);
  for (int i = 0; i < n;) {
    int x = 1 - y;
    if (mp[x] == 0) out("NO");
    if (mp[x] > mp[y]) {
      int take = min(mp[x], k);
      for (int j = i; j < i + take; j++) {
        ans[j] = x;
      }
      mp[x] -= take;
      i += take;
    } else {
      ans[i] = x;
      mp[x]--;
      i++;
    }
    y = x;
  }
  for (int i = 0; i < n; i++) {
    char c = (ans[i] == 0) ? 'G' : 'B';
    cout << c;
  }
  cout << endl;
  return 0;
}
