#include <bits/stdc++.h>

using namespace std;

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

    cout << fixed << setprecision(12);
    cout << (double) (abs(px - qx) + abs(py - qy)) / 2.0 << endl;
    return 0;
}