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