#include <iostream>
using namespace std;
#include <cmath>
#include <iomanip>
int main(int argc, char** argv)
{
int i,n;
double sum;
cin>>n;
for(i=1,sum=0;i<=n;i++)
{
sum=sum+1/(3i-2);
}
cout<<setiosflags(ios::fixed)<<setprecision(2)<<sum<<endl;
return 0;
}
