#include<iostream>
#include<iomanip>
#include<cmath>
#include<string>
#include<algorithm>
#include<vector>
#include<math.h>

using namespace std;
int main(){
    double Px, Py, Qx, Qy;
    cin >> Px >> Py >> Qx >> Qy;
    double Dx, Dy;
    double ans;
    if(Px - Qx >= 0){
        Dx = Px - Qx;
    }else{
        Dx = Qx - Px;
    }
    if(Py - Qy >= 0){
        Dy = Py - Qy;
    }else{
        Dy = Qy - Py;
    }
    ans = Dx /2 + Dy / 2;
    cout <<fixed << ans << endl;
    return 0;
}