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 C++ (GCC 9.2.1) (judge verdict: Accepted)

#include <bits/stdc++.h>
using namespace std;

#define repi(i, a, n) for (int i = a; i < (int)(n); ++i)
#define rep(i, n) repi(i, 0, n)
#define all(v) begin(v), end(v)

using ll = long long;

int main() {
  // freopen("1.txt", "rb", stdin);
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);

  int N;
  cin >> N;

  vector<int> f(N);
  for (int i = 1; i < N; ++i) {
    for (int j = 1; 1LL * j * i < N; ++j) {
      ++f[i * j];
    }
  }

  ll sum = 0;
  for (int a : f) {
    sum += a;
  }
  cout << sum << '\n';

  return 0;
}