#include <bits/stdc++.h>

using namespace std;

const int N = 110;
double a[N], c[N];

int main() {
    a[1] = 2;
    c[1] = 1;

    a[2] = 3;
    c[2] = 2;

    double s = a[1] / c[1] + a[2] / c[2];
    for (int i = 3; i <= 20; i++) {
        a[i] = a[i - 1] + a[i - 2];
        c[i] = c[i - 1] + c[i - 2];
        s += a[i] / c[i];
    }
    cout << s << endl;
    return 0;
}