#include <stdio.h>
#include <math.h>
double px,py;
double qx,qy;
double absolute(double x){
        if(x<0){
                return -x;
        }else{
                return x;
        }
}
int main(){
        double T =0; //T means time.
        double xave=0, yave=0;
        scanf("%lf %lf", &px, &py);
        scanf("%lf %lf", &qx, &qy);
        xave = absolute(px-qx);
        yave = absolute(py-qy);
        T = (xave + yave) /2 ;
        printf("%lf\n", T);
        return 0;
}