#include <iostream>

using namespace std;

double series(int n);

double series(int n)
{
    if (n <= 0)
        return 0;
    else
        return (double) (n/(2.0*n+1.0)) + series(n-1);
}

int main(int argc, const char *argv[])
{
    long x;
    cout << "enter a number to see the sum of the series (negative to quit)" << endl;
    do {
        cout << ">> ";
        cin >> x;
        if (x >= 0)
            cout << "series(" << x << ") = " << series(x) << endl;
    } while (x >= 0);
    return 0;
}