#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); }