#include<bits/stdc++.h>
using namespace std;
#define rep(i,a,b) for(int i=a;i<b;i++)



//-----------------------------------------------------------------------------------
int main() {
    int x; cin >> x;

    double ans = 0;
    rep(i, 1, 101010101) ans += 1.0 / (x + i) / (x + i);
    printf("%.10f\n", ans);
}