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

int main()
{
    int p[4];
    for (auto& x : p) cin >> x;

    cout << (abs(p[0]) + abs(p[1]) + abs(p[2]) + abs(p[3])) / 2.0 << endl;

    return 0;
}