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: Runtime Error)

import math

N = int(input())

ans = 0
for A in range(1, N):
    for B in range(1, N):
        if A >= math.sqrt(N) and B >= math.sqrt(N):
            ans2 = ans + 1 - A*B
            return

        if A * B < N:  # 大きさ比較なので計算早いか？
            ans += 1
        else:
            break  # 値大きくなったらもう次のAに

print(ans + ans2)