#include <iostream>
#include <iomanip>
#include <cmath>
#define repeat_from(i,m,n) for (int i = (m); (i) < (n); ++(i))
using namespace std;
int main() {
    long double x; cin >> x;
    long double y = 0;
    repeat_from (n,1,1000000) y += pow(x+n, -2);
    cout << setprecision(12) << y << endl;
    return 0;
}