/* -*- coding: utf-8 -*- * * 454.cc: No.454 ι2δΉε - yukicoder */ #include<cstdio> #include<cstdlib> #include<cstring> #include<cmath> #include<iostream> #include<string> #include<vector> #include<map> #include<set> #include<stack> #include<list> #include<queue> #include<deque> #include<algorithm> #include<numeric> #include<utility> #include<complex> #include<functional> using namespace std; /* constant */ const int MAX_N = 2000000; /* typedef */ typedef long double ld; /* global variables */ /* subroutines */ /* main */ int main() { ld x; cin >> x; ld sum = 0.0; for (int n = 1; n <= MAX_N; n++) { ld nx = x + n; sum += (ld)1.0 / (nx * nx); } sum += (ld)1.0 / MAX_N; printf("%.12Lf\n", sum); return 0; }