#include <bits/stdc++.h>
using namespace std;
ofstream fo("test.out");
ifstream fi("test.inp");
int main() {
  long long x1, y1, x2, y2;
  cin >> x1 >> y1 >> x2 >> y2;
  cout << max(abs(x1 - x2), abs(y1 - y2));
}
