<?php
    list($x1,$y1)=explode(" ",trim(fgets(STDIN)));
    list($x2,$y2)=explode(" ",trim(fgets(STDIN)));

    $ans=0;

    if($x1>=$x2){
        $time_x=($x1-$x2)/2;
    }else{
        $time_x=($x2-$x1)/2;
    }

    if($y1>=$y2){
        $time_y=($y1-$y2)/2;
    }else{
        $time_y=($y2-$y1)/2;
    }

    $ans=$time_x+$time_y;
    echo $ans.PHP_EOL;