#include <iostream>
#include <iomanip>
using namespace std;

int main() {
    int px, py, qx, qy;

    cin >> px >> py >> qx >> qy;

    int dist = abs(px - qx) + abs(py - qy);
    cout << fixed << setprecision(10) << (double)dist / 2 << endl;

    return 0;
}