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> 
#include <chrono>
using namespace std;
 
template<class T> inline string toString(T x) {ostringstream sout;sout<<x;return sout.str();}
 
 
//typedef
//------------------------------------------
typedef pair<int, int> PII;
typedef long long LL;
typedef pair<LL, LL> PLL;
 
//container util
//------------------------------------------
#define PB emplace_back
#define MP make_pair 
#define SZ(a) int((a).size())
//repetition
//------------------------------------------
#define FOR(i,a,b) for(LL i=(a);i<(b);++i)
#define REP(i,n)  FOR(i,0,n)
#define SORT(c) sort((c).begin(),(c).end())
#define ALL(a)  (a).begin(),(a).end()
//constant
//--------------------------------------------
//clear memory
#define CLR(a) memset((a), 0 ,sizeof(a))

int main(){
    LL N;
    cin>>N;
    LL res=0;
    FOR(A,1,N){
        res+=(N-1)/A;
    }
    cout<<res<<endl;
    return 0;
}