#include <bits/stdc++.h>
using namespace std;
void print(vector<int> v) {
  for (int i = 0; i < v.size(); ++i) {
    cout << v[i] << ' ';
  }
  cout << "\n";
}
int main() {
  int n, k, a, b;
  cin >> n >> k >> a >> b;
  if (a >= b) {
    int x = a / k;
    int r = a % k;
    int t, u, v;
    int y, r2, unum;
    if (r == 0) {
      if (b < x - 1) {
        cout << "NO";
        return 0;
      }
    } else {
      if (b < x) {
        cout << "NO";
        return 0;
      }
    }
    if (b >= x * k) {
      t = b - x * k;
      u = x;
      v = 0;
      unum = k;
    } else {
      y = b / x;
      r2 = b % x;
      t = 0;
      unum = y + 1;
      u = r2;
      v = x - r2;
    }
    for (int i = 0; i < t; ++i) cout << 'B';
    while (x--) {
      for (int i = 0; i < k; ++i) cout << 'G';
      if (u > 0) {
        for (int i = 0; i < unum; ++i) cout << 'B';
        u--;
      } else {
        for (int i = 0; i < unum - 1; ++i) cout << 'B';
        v--;
      }
    }
    for (int i = 0; i < r; ++i) cout << 'G';
    cout << '\n';
  } else {
    int x = b / k;
    int r = b % k;
    int t, u, v;
    int y, r2, unum;
    if (r == 0) {
      if (a < x - 1) {
        cout << "NO\n";
        return 0;
      }
    } else {
      if (a < x) {
        cout << "NO\n";
        return 0;
      }
    }
    if (a >= x * k) {
      t = a - x * k;
      u = x;
      v = 0;
      unum = k;
    } else {
      y = a / x;
      r2 = a % x;
      t = 0;
      unum = y + 1;
      u = r2;
      v = x - r2;
    }
    for (int i = 0; i < t; ++i) cout << 'G';
    while (x--) {
      for (int i = 0; i < k; ++i) cout << 'B';
      if (u > 0) {
        for (int i = 0; i < unum; ++i) cout << 'G';
        u--;
      } else {
        for (int i = 0; i < unum - 1; ++i) cout << 'G';
        v--;
      }
    }
    for (int i = 0; i < r; ++i) cout << 'B';
    cout << '\n';
  }
  return 0;
}
