#include<cstdio>
#include<cstdlib>
#include<algorithm>
#include<iostream>
#include<queue>
#include<vector>
#include <bitset>
#include <cmath>
#include <limits>
#include <iostream>
using namespace std;
#define INF 100000
#define MAX 100000
#define MOD 1000000007
typedef long long ll;
typedef pair<int,int> P;
typedef pair< pair<int,int>, int> p; 
#define bit(n,k) ((n>>k)&1) /*nのk bit目*/
#define rad_to_deg(rad) (((rad)/2/M_PI)*360)

int main(){
    int px,py,qx,qy;
    cin>>px>>py>>qx>>qy;
    double ans=0;
    ans+=abs(px-qx);
    ans+=abs(py-qy);
    printf("%.2f\n",ans/2);
}