#include <stdio.h>
#include <stdlib.h>

int main(void){
    int Px,Py,Qx,Qy;
    
    scanf("%d%d%d%d",&Px,&Py,&Qx,&Qy);
    printf("%lf",(double)(abs(Px-Qx)+abs(Py-Qy))/2);
}