#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 xx = abs(x1 - x2);
  double yy = abs(y1 - y2);
  cout << (xx + yy) / 2.0 << endl;
  return 0;
}