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

int main() {
    double p_x, p_y, q_x, q_y;
    cin >> p_x >> p_y;
    cin >> q_x >> q_y;
    double pq_dif;
    pq_dif = fabs(p_x - q_x) + fabs(p_y - q_y);
    double ans;
    ans = pq_dif / 2.0;
    cout << ans << endl;
    return 0;
}