#include<bits/stdc++.h>
int main(void) {
	long double a, b, c, d;
	scanf(" %Lf %Lf %Lf %Lf", &a, &b, &c, &d);
	long double s = (a + b + c + d) / 2.0;
	printf("%.8f\n", sqrt((s-a)*(s-b)*(s-c)*(s-d)));
    return 0;
}