#define _USE_MATH_DEFINES
#include <cstdio>
#include <iostream>
#include <sstream>
#include <fstream>
#include <iomanip>
#include <algorithm>
#include <cmath>
#include <complex>
#include <string>
#include <vector>
#include <list>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <bitset>
#include <numeric>
#include <limits>
#include <climits>
#include <cfloat>
#include <functional>
#include <iterator>
using namespace std;

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

    int dist = abs(px - qx) + abs(py - qy);
    cout << (dist / 2);
    if(dist % 2 != 0)
        cout << ".5";
    cout << endl;

    return 0;
}