#include <stdio.h>
#include <stdlib.h>
int main(){
    //input
    int Px,Py;
    int Qx,Qy;
    scanf("%d %d",&Px,&Py);
    scanf("%d %d",&Qx,&Qy);

    //length
    int l = abs(Px-Qx)+abs(Py-Qy);

    //min time
    double t = 0.5*l;

    //output
    printf("%f\n",t);
    return 0;
}