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