#include <bits/stdc++.h>
using namespace std;
long long MCD(long long a, long long b) {
  long long x, y;
  if (a < b) {
    x = b;
    y = a;
  } else {
    x = a;
    y = b;
  }
  if (y == 0) {
    return x;
  } else {
    return MCD(x % y, y);
  }
}
int main() {
  ios_base ::sync_with_stdio(false);
  long long n, k, a, b;
  cin >> n >> k >> a >> b;
  if (a != 0 && b != 0) {
    if (b <= (k) * (a + 1) && a <= (k) * (b + 1)) {
      if (a > b) {
        while (a - b >= k && b > 0 && a > 0) {
          for (int i = 0; i < k; i++) {
            cout << "G";
          }
          if (b > 0) {
            cout << "B";
          }
          a = a - k;
          b--;
        }
        for (int i = 1; i <= (a - b); i++) {
          cout << "G";
          a--;
        }
        while (a > 0) {
          a--;
          cout << "G";
          if (b > 0) {
            cout << "B";
            b--;
          }
        }
      } else {
        if (a < b) {
          while (b - a >= k && a > 0 && b > 0) {
            for (int i = 0; i < k; i++) {
              cout << "B";
            }
            cout << "G";
            b = b - (k);
            a--;
          }
          for (int i = 1; i <= (b - a); i++) {
            cout << "B";
            b--;
          }
          while (b > 0) {
            b--;
            cout << "B";
            if (a > 0) {
              cout << "G";
              a--;
            }
          }
        } else {
          while (a > 0) {
            a--;
            cout << "GB";
          }
        }
      }
    } else {
      cout << "NO" << endl;
    }
  } else {
    if (a <= k && b <= k) {
      if (a != 0) {
        for (int i = 0; i < a; i++) {
          cout << "G";
        }
      } else {
        for (int i = 0; i < b; i++) {
          cout << "B";
        }
      }
    } else {
      cout << "NO" << endl;
    }
  }
}
