#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>
#include <iomanip>
#include <stack>
#include <algorithm>
#include <string>
#include <map>
#include <iterator>
#include <set>
#include <queue>

using namespace std;


int main() {

    double Px, Py, Qx, Qy;
    cin >> Px >> Py >> Qx >> Qy;

    double result = (abs(Px - Qx) + abs(Py - Qy)) / 2.0;

    cout << result << endl;

    return 0;
}