#include <bits/stdc++.h>
using namespace std;
int main() {
  int x1, y1, x2, y2, otvet;
  cin >> x1 >> y1 >> x2 >> y2;
  otvet = abs(x1 - x2);
  if (abs(y1 - y2) > otvet) otvet = abs(y1 - y2);
  cout << otvet << endl;
  return 0;
}
