#include <iostream>
#include <iomanip>

using namespace std;

int main()
{
    double sum = 0;
    double x;
    cin >> x;
    int sgn = -1, i = 1;;
    do
    {
        sum += sgn * (x / i);
        sgn *= -1;
        i++;
        cin >> x;
    }
    while (x > 0);
    cout << fixed << setprecision(6);
    cout << sum;

    return 0;
}