#include <iostream>
#include <stdio.h>
#include <algorithm>
#include <math.h>
#include <utility>
#include <vector>
using namespace std;


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

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

    cout << (double)(abs(px-qx)+abs(py-qy))/2 << endl;
    return 0;
}