백준 #8370번 Plane

import java.util.Scanner;
public class Main {
	public static void main(String[] args) {
		
		Scanner sc = new Scanner(System.in);
		
		int n1 = sc.nextInt();
		int k1 = sc.nextInt();
		int n2 = sc.nextInt();
		int k2 = sc.nextInt();
		int total = n1 * k1 + n2 * k2;
		
		System.out.println(total);
		sc.close();
	}
}