import sys
input = lambda : sys.stdin.readline().rstrip()


sys.setrecursionlimit(2*10**5+10)
write = lambda x: sys.stdout.write(x+"\n")
debug = lambda x: sys.stderr.write(x+"\n")
writef = lambda x: print("{:.12f}".format(x))

import math
def f(x):
    return 1/x/x
x = float(input())
ans = 0
for v in range(1,10**7)[::-1]:
    ans += f(x+v)
ans += 1/(x+10**7)
print(ans)