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 Python (3.8.2) (judge verdict: Accepted)

"""O(N^0.5) solution"""

N = int(input())
N -= 1

ans = 0
n = int(N**.5)
for x in range(1, n+1):
  # (x,x)
  ans += 1
  # (x,y)
  ans += N//x - x
  # (y,x)
  ans += N//x - x

print(ans)