#include<bits/stdc++.h>
using namespace std;
int main(){
    int s1,s2,t1,t2;
    cin>>s1>>s2>>t1>>t2;
    cout<<max(abs(t1-s1),abs(t2-s2))<<endl;
    return 0;
}

/**************************************************************
	Problem: 1575
	User: admin
	Language: C++
	Result: Accepted
	Time:49 ms
	Memory:2072 kb
****************************************************************/