#include<stdio.h>
int main()
{
    int n;
    scanf("%d", &n);
    int i, s=1;
    int tmp = 0;
    double sum = 0;
    for (i = 1; i <= n; i++)
    {
        tmp += s*(2*i-1);
        sum += 1.0/tmp;
        s = -s;
    }
    printf("%.3lf\n", sum);
}