#include <stdio.h>

int main()
{
	int W, X, Y, Z;
	scanf("%d %d %d %d", &X, &Y, &Z, &W);
	printf("%.6Lf\n", (long double)X / 2.0 - (long double)Y * Z / 2.0);
	fflush(stdout);
	return 0;
}