#include<stdio.h>
int main(void)
{
double Px,Py,Qx,Qy,length;
scanf("%lf %lf %lf %lf",&Px,&Py,&Qx,&Qy);

if(Px>Qx){
        length=Px-Qx;
}
else{
        length=Qx-Px;
}

if(Py>Qy){
        length+=Py-Qy;
}
else{
        length+=Qy-Py;
}
length/=2;

printf("%f",length);
return 0;
}