Problem:
Score : 300 points

Problem Statement

Given is a positive integer N.
How many tuples (A,B,C) of positive integers satisfy A \times B + C = N?

Constraints

2 \leq N \leq 10^6

All values in input are integers.

Input

Input is given from Standard Input in the following format:

N

Output

Print the answer.

Sample Input 1

3

Sample Output 1

3

There are 3 tuples of integers that satisfy A \times B + C = 3: (A, B, C) = (1, 1, 2), (1, 2, 1), (2, 1, 1).

Sample Input 2

100

Sample Output 2

473

Sample Input 3

1000000

Sample Output 3

13969985

Solution in Ruby (2.7.1) (judge verdict: Accepted)

io = DATA.eof? ? STDIN : DATA
n=io.gets.to_i

Amax=n
D = Array.new(Amax + 2,0)
D[1]=1
(2 .. n).each do |i|
  s = i
  while s < D.size
    if s==i
      D[s]+=2
    else
      D[s]+=1
    end
    s += i
  end
end
puts D[1..(n-1)].sum

__END__