#include<iostream>
#include<vector>
#include<algorithm>
#include<cstdio>
#include<cmath>
#include<string>
using namespace std;
int main(){
    long long PX,PY,QX,QY,X,Y,num,n;
    float ans;
    cin >> PX >> PY >> QX >> QY;
    X = QX-PX;
    Y = QY-PY;
    if(X<0) X*=-1;
    if(Y<0) Y*=-1;
    n = X+Y;
    if(n%2==1)  cout << ((n/2)+0.5);
    else cout << (n/2);
    return 0;
}