#include <bits/stdc++.h>
using namespace std;
const int maxm = 1e6 + 10;
int main() {
  int x1, y1, x2, y2;
  while (scanf("%d%d%d%d", &x1, &y1, &x2, &y2) != EOF) {
    int m = max(abs(x1 - x2), abs(y1 - y2));
    printf("%d\n", m);
  }
  return 0;
}
