import java.util.*;

public class Run {
    public static void main (String arg[]) {
        Scanner scan = new Scanner(System.in);
        String ans = "NO";
        long HW = scan.nextLong() * scan.nextLong();
        long N = scan.nextLong();
        long K = scan.nextLong();
        if (HW%N==K || HW==K) ans = "YES";
        System.out.println(HW);
    }
}