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;
using ll = long long;
using vi = vector<int>;
using vc = vector<char>;
using vs = vector<string>;
using vl = vector<ll>;
using vb = vector<bool>;
using vvi = vector<vector<int>>;
using vvl = vector<vector<ll>>;
using vvc = vector<vector<char>>;
using vvs = vector<vector<string>>;
using vvb = vector<vector<bool>>;
#define rep(i, n) for(int i = 0; i < n; i++)
const ll mod = 1000000007;
int dx[4]={1,0,-1,0};
int dy[4]={0,1,0,-1};

int main() {
  int N;
  cin >> N;
  ll count = 0;
  for(int i = 1; i <= N; i++) {
    if( N % i != 0) {
      int b = N / i;
      count += b;
    }
    else {
      count += N / i - 1;
    }
  }
  cout << count << endl;
}