#include <algorithm>
#include <cstdio>
#include <iostream>
#include <map>
#include <cmath>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <vector>
#include <stdlib.h>
#include <stdio.h>
#include <bitset>
using namespace std;
#define FOR(I,A,B) for(int I = (A); I < (B); ++I)
typedef long long ll;

int main(){
  double x1, y1, x2, y2;
  cin >> x1 >> y1 >> x2 >> y2;
  double dy = fabs(y1 - y2);
  double dx = fabs(x1 - x2);
  double ans = (dy + dx) / 2.0;
  printf("%lf\n", ans);
  return 0;
}