#include<bits/stdc++.h>
using namespace std;

int main(){
    int px,py,qx,qy;
    int ans=0.0;
    cin >> px >> py >> qx >> qy;

    ans+=(px>qx)?px-qx:qx-px;
    ans+=(py>qy)?py-qy:qy-py;

    cout << ans/2;
    if(ans%2!=0) cout << ".5";
    cout << endl;

    return 0;
}