#include<stdio.h>
int main(void)
{
    double Px,Py,Qx,Qy,a;
    scanf("%lf %lf  %lf %lf",&Px,&Py,&Qx,&Qy);
    if(Px>Qx){
        a=Px-Qx;
    }
    else{
        a=Qx-Px;
    }
    if(Py>Qy){
        a+=Py-Qy;
    }
    else{
        a+=Qy-Py;
    }
    a/=2;
    printf("%.10f\n",a);
    return 0;
}