#include<stdio.h>
int main()
{
    double n;
    int i,j;
    double s=0;
    scanf("%d",&n);
    for(i=1,j=1;i<=n;i++,j*=-1)
    {
        s+=j*(2*i-1)*(2*i+1)/(2*i*2*i*1.0);
    }                                         
    printf("%lf\12",s);
    return 0;
}