/* -*- coding: utf-8 -*-
 *
 * 410.cc: No.410 出会い - yukicoder
 */

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<iostream>
#include<string>
#include<vector>
#include<map>
#include<set>
#include<stack>
#include<list>
#include<queue>
#include<deque>
#include<algorithm>
#include<numeric>
#include<utility>
#include<complex>
#include<functional>
 
using namespace std;

/* constant */

/* typedef */

/* global variables */

/* subroutines */

/* main */

int main() {
  int px, py, qx, qy;
  cin >> px >> py >> qx >> qy;

  int d = abs(px - qx) + abs(py - qy);
  if (d & 1) printf("%d.5\n", d / 2);
  else printf("%d\n", d / 2);
  return 0;
}