#include <bits/stdc++.h>
#define rep(i, a, n) for(int i = a; i < n; i++)
#define repb(i, a, b) for(int i = a; i >= b; i--)
// #define FOR(i, a) for(auto& i : a)
#define all(a) a.begin(), a.end()
#define int long long
using namespace std;

signed main(){
    double x[2], y[2], ans;
    rep(i, 0, 2) cin >> x[i] >> y[i];
    ans = fabs(x[0] - x[1]) + fabs(y[0] - y[1]);
    ans = (double)ans / (double)2.0;
    printf("%.8lf\n", ans);
}