#include <bits/stdc++.h>
using namespace std;
int main() {
  int x1, x2, y1, y2;
  scanf("%d %d", &x1, &y1);
  scanf("%d %d", &x2, &y2);
  printf("%d", max(abs(x1 - x2), abs(y1 - y2)));
  return 0;
}
