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); } }