#include <bits/stdc++.h>

using namespace std;

int main() {
    int px, py, qx, qy;
    cin >> px >> py >> qx >> qy;
    double ans = (abs(px - qx) + abs(py - qy)) / 2.0;
    cout << fixed << setprecision(12);
    cout << ans << endl;
    return 0;
}