#include <cstdio>

using namespace std;

const int MAXN = 1e6 + 10;
const double LIMIT = 1.64493;

double A[MAXN] = {0, 1};

void init()
{
    for (int i = 2; i < MAXN; i++)
    {
        A[i] = A[i - 1] + 1.0 / (i * 1.0 * i);
    }
}
long long n;

int main()
{
    init();

    while (~scanf("%lld", &n))
    {
        if (n < MAXN)
        {
            printf("%.5f\n", A[n]);
        }
        else
        {
            printf("%.5f\n", LIMIT);
        }
    }
    return 0;
}