//分子序列求和
#include<iostream>
#include<iomanip>

using namespace std;

int main(){
    int n;
    double a=2,b=1,c,d;
    cin >>n;
    
    if (n > 100)
    {
        cout <<"输入的数字太大了!"<<endl;
        return 1;
    }

    double sum=0;
    for (int i = 1; i <= n; i++)
    {
        sum += a/b;
        c = a;
        d = b;
        a = a+d;
        b = c;
    }
    cout <<fixed<<setprecision(2)<<sum<<endl;
    return 0;
}