import java.util.Scanner;

public class Main{

	static Scanner s=new Scanner(System.in);

	public static void main(String __[]){
		input();
		solve();
	}

	private static void input(){
	}

	private static void solve(){
		long a=s.nextLong()*s.nextLong(),c=s.nextLong(),d=s.nextLong();
		a%=d;
		a*=c;
		a%=d;
		System.out.println(a);
	}
}