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