# include <iostream>
# include <iomanip>
# include <algorithm>

using namespace std;


int main()
{
    double Px, Py;
    double Qx, Qy;

    cin >> Px >> Py >> Qx >> Qy;

    cout << (abs(Px-Qx)+abs(Py-Qy))/2 << endl;

}