#include <bits/stdc++.h>

using namespace std;

int main() {
    int px, py, qx, qy;
    
    cin >> px >> py >> qx >> qy;
    
    double tm = (abs(px - qx) + abs(py - qy)) / 2.0;
    
    cout << setprecision(10) << tm << endl;
    
    return 0;
}