// #define _GLIBCXX_DEBUG
#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); ++i)
int main() {
    // Input
    double PX, PY, QX, QY;
    cin >> PX >> PY >> QX >> QY;
    
    // Output
    cout << fixed << setprecision(20);
    cout << (abs(PX - QX) + abs(PY - QY)) / 2.0 << endl;
}