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