#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);
        if(qx<0) qx = -qx;
        if(qy<0) qy = -qy;
        if(px<0) px = -px;
        if(py<0) py = -py;
        xave = ( px + qx ) / 2;
        xave = absolute(xave);
        yave = ( py + qy ) / 2;
        yave = absolute(yave);
        T = xave +yave;
        printf("%lf\n", T);
        return 0;
}