#include <iostream>
#include <algorithm>
#include <string>
#include <cstdio>
using namespace std;

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

    double t = (abs(px - qx) + abs(py - qy)) / 2.0;
    printf("%f\n", t);

    return 0;
}