#include <bits/stdc++.h>
using namespace std;
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
const long long maxn = 2e5 + 5, N = 2e6 + 5, SQ = 600, base = 800287,
                mod = 998244353, INF = 1e12 + 5, lg = 20;
const long double eps = 1e-4;
long long cnt[maxn];
int32_t main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  long long n, k, a, b;
  cin >> n >> k >> a >> b;
  char x = 'G', y = 'B';
  if (a < b) {
    swap(a, b);
    swap(x, y);
  }
  if ((a + b) / (b + 1) > k) {
    return cout << "NO\n", 0;
  }
  long long d = min(a, b + 1);
  for (long long i = 0; i < d; ++i) {
    cnt[i] = 1;
    a--;
  }
  long long id = 0;
  while (a > 0) {
    long long r = min(a, k - 1);
    cnt[id] += r, a -= r;
    id++;
  }
  for (long long i = 0; i < d; ++i) {
    for (long long j = 0; j < cnt[i]; ++j) {
      cout << x;
    }
    if (b) {
      cout << y;
      b--;
    }
  }
  cout << '\n';
}
