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 Fortran (GNU Fortran 9.2.1) (judge verdict: Accepted)

      PROGRAM piyo
      IMPLICIT NONE
      INTEGER,PARAMETER :: p = 8
      integer(p) :: n,a,b,c ,nn=10**6
      integer(p) :: t(0:10**6)
      
      read*,n
      
      do a = 1,nn
        do b = 1, nn/a
          if( a*b>10**6 )cycle
          
          t(a*b) = t(a*b) + 1
        end do
      end do
      
      print*,sum(t(1:n-1))
      
      
      
      stop
      !debugg
      
      
      
      END PROGRAM