import std.stdio;
import std.algorithm;

void main()
{
	long x1, x2, y1, y2;

	scanf("%lld%lld%lld%lld", &x1, &y1, &x2, &y2);

	long dx = (x2 - x1) / 2;
	long dy = (y2 - y1) / 2;
	printf("%lld\n", (dx + 1) * (dy + 1) + dx * dy);
}