#include <stdio.h>

int main()
{
	int a = 0; 
	int b = 0; 
	int c = 0;
	int d = 0; 
	scanf("%d%d%d%d", &a, &b, &c, &d); 
	printf("%d\n", (a + b - c) * d); 
	return 0; 
}