#include<cstdio>

#include<algorithm>

#include<cstring>

#define Max(a,b,c,d) max(max(a,b),max(c,d))

#define Min(a,b,c,d) min(min(a,b),min(c,d))

using namespace std;

double a,b,c,d,l,r=1000000000,mid;

bool check(double lim)

{

    double max_x=Max((a+lim)*(d+lim),(a-lim)*(d-lim),(a+lim)*(d-lim),(a-lim)*(d+lim));

    double min_x=Min((a+lim)*(d+lim),(a-lim)*(d-lim),(a+lim)*(d-lim),(a-lim)*(d+lim));

    double max_y=Max((b+lim)*(c+lim),(b+lim)*(c-lim),(b-lim)*(c-lim),(b-lim)*(c+lim));

    double min_y=Min((b+lim)*(c+lim),(b+lim)*(c-lim),(b-lim)*(c-lim),(b-lim)*(c+lim));

    if(max_x-min_y>=0&&min_x-max_y<=0)

	return 1;

    else return 0;

}

int main()

{

    scanf("%lf%lf%lf%lf",&a,&b,&c,&d);

    for (int i=1;i<=100;i++)

    {

	mid=(l+r)/2;

	if (check(mid)) r=mid;

	else l=mid;

    }

    printf("%.10lf",l);

    return 0;

}