#include <stdio.h>

void main(void){
    int aX,aY;
    scanf("%d %d",&aX,&aY);
    int bX,bY;
    scanf("%d %d",&bX,&bY);

    double positionX = aX - bX ;
    if(positionX < 0){
        positionX *= -1 ;
    }
    double positionY = aY - bY ;
    if(positionY < 0){
        positionY *= -1 ;
    }

    double shortestTime = (positionX + positionY) / 2;

    printf("%f",shortestTime);

}