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>
typedef long long int ll;
using namespace std;
#define sz 1000009
#define mx 10000000000008
#define mod 1000000007
ll n,t,arr[sz],brr[sz],crr[sz],j;
vector<ll>v;
char c[sz];
int main()
{
    for(int i=1;i<=1000000;i++)
    {
        for(int j=i;j<=1000000;j+=i)
            arr[j]++;
    }
    for(int i=1;i<=1000000;i++)
        arr[i]+=arr[i-1];
    scanf("%lld",&n);
    printf("%lld\n",arr[n-1]);
    return 0;
}