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

int main() {
    int Px, Py;
    int Qx, Qy;
    cin >> Px >> Py;
    cin >> Qx >> Qy;
    cout << (abs(Px-Qx)+abs(Py-Qy))/2.0 << endl;
    return 0;
}