#include <bits/stdc++.h>
using ll = long long;
using llu = long long unsigned;

using namespace std;

int main() {
    int Px, Py, Qx, Qy;
    cin >> Px >> Py >> Qx >> Qy;
    double result = (abs((double)Px-Qx) + abs(Py-Qy))/2;
    cout << result << endl;
    return 0;
}