#include <cmath>
#include <iostream>

int main() {
    using namespace std;
    int px, py, qx, qy;
    cin >> px >> py >> qx >> qy;
    int d = abs(px - qx) + abs(py - qy);
    cout << d / 2 << '.' << (d % 2 == 0 ? 0 : 5) << endl;
}