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 ld        = long double;
using vl        = vector<ll>;
using vd        = vector<ld>;
using vs        = vector<string>;
using vb        = vector<bool>;
using vvl       = vector<vector<ll>>;
using vvd       = vector<vector<ld>>;
using vvs       = vector<vector<string>>;
using vvb       = vector<vector<bool>>;
using pll       = pair<ll, ll>;
constexpr ll mod= 1e9 + 7;
#define ALL(x) (x).begin(), (x).end()
#define REP(i, n) for(ll(i)= 0; (i) < (n); (i)++)
#define REPS(i, n) for(ll(i)= 1; (i) <= (n); (i)++)
#define RREP(i, n) for(ll(i)= (n - 1); (i) >= 0; (i)--)
#define RREPS(i, n) for(ll(i)= (n); (i) > 0; (i)--)
#define pb push_back
#define eb emplace_back
#define mp make_pair
#define fi first
#define se second
#define UNIQUE(v) v.erase(unique(ALL(v)), v.end())
#define YES(n) ((n) ? "YES" : "NO")
#define Yes(n) ((n) ? "Yes" : "No")
#define yes(n) ((n) ? "yes" : "no")

template <class T>
inline void chmin(T& a, T b) {
    if(a > b) { a= b; }
}

template <class T>
inline void chmax(T& a, T b) {
    if(a < b) { a= b; }
}

signed main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(12);
    ll N;
    cin >> N;
    ll ans = 0;
    REPS(i,N){
        for(int l = i;l*i <= N;l++){
            if(l == i){
                if(l*i <= N-1)ans++;
            }else{
                if(l*i <= N-1)ans+=2;
            }
        }
    }
    cout << ans << "\n";
}