#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int x1, y1, x2, y2;
  cin >> x1 >> y1 >> x2 >> y2;
  cout << max(abs(x1 - x2), abs(y1 - y2)) << "\n";
}
