#include <bits/stdc++.h>
#define rep(i,n) for (int i=0; i<(n); ++i)
using namespace std;
using ll=long long;
int main(){
    double px,py,qx,qy;
    cin>>px>>py>>qx>>qy;
    cout<<fixed<<setprecision(15)<<abs(px-qx)/2+abs(py-qy)/2<<endl;
}