Language: C++

#include <bits/stdc++.h>
#define int auto

using namespace std; 
  
void SieveOfEratosthenes(int n, bool prime[], 
                         bool primesquare[], int a[]) 
{ 
    // Create a boolean array "prime[0..n]" and 
    // initialize all entries it as true. A value 
    // in prime[i] will finally be false if i is 
    // Not a prime, else true. 
    for (int i = 2; i <= n; i++) 
        prime[i] = true; 
  
    // Create a boolean array "primesquare[0..n*n+1]" 
    // and initialize all entries it as false. A value 
    // in squareprime[i] will finally be true if i is 
    // square of prime, else false. 
    for (int i = 0; i <= (n * n + 1); i++) 
        primesquare[i] = false; 
  
    // 1 is not a prime number 
    prime[1] = false; 
  
    for (int p = 2; p * p <= n; p++) { 
        // If prime[p] is not changed, then 
        // it is a prime 
        if (prime[p] == true) { 
            // Update all multiples of p 
            for (int i = p * 2; i <= n; i += p) 
                prime[i] = false; 
        } 
    } 
  
    int j = 0; 
    for (int p = 2; p <= n; p++) { 
        if (prime[p]) { 
            // Storing primes in an array 
            a[j] = p; 
  
            // Update value in primesquare[p*p], 
            // if p is prime. 
            primesquare[p * p] = true; 
            j++; 
        } 
    } 
} 
  
// Function to count divisors 
int countDivisors(int n) 
{ 
    // If number is 1, then it will have only 1 
    // as a factor. So, total factors will be 1. 
    if (n == 1) 
        return 1; 
  
    bool prime[n + 1], primesquare[n * n + 1]; 
  
    int a[n]; // for storing primes upto n 
  
    // Calling SieveOfEratosthenes to store prime 
    // factors of n and to store square of prime 
    // factors of n 
    SieveOfEratosthenes(n, prime, primesquare, a); 
  
    // ans will contain total number of distinct 
    // divisors 
    int ans = 1; 
  
    // Loop for counting factors of n 
    for (int i = 0;; i++) { 
        // a[i] is not less than cube root n 
        if (a[i] * a[i] * a[i] > n) 
            break; 
  
        // Calculating power of a[i] in n. 
        int cnt = 1; // cnt is power of prime a[i] in n. 
        while (n % a[i] == 0) // if a[i] is a factor of n 
        { 
            n = n / a[i]; 
            cnt = cnt + 1; // incrementing power 
        } 
  
        // Calculating the number of divisors 
        // If n = a^p * b^q then total divisors of n 
        // are (p+1)*(q+1) 
        ans = ans * cnt; 
    } 
  
    // if a[i] is greater than cube root of n 
  
    // First case 
    if (prime[n]) 
        ans = ans * 2; 
  
    // Second case 
    else if (primesquare[n]) 
        ans = ans * 3; 
  
    // Third case 
    else if (n != 1) 
        ans = ans * 4; 
  
    return ans; // Total divisors 
} 
  
// Driver Program 
int main() 
{ 
   int n; cin>>n;
   int num=0;
   for(int i=1;i<n;i++)
      num+=countDivisors(n-i);
  cout<<num<<endl;
  return 0;
   
} 

Compiler output:
a.cc:2:13: warning: use of 'auto' in parameter declaration only available with '-std=c++20' or '-fconcepts'
    2 | #define int auto
      |             ^~~~
a.cc:6:26: note: in expansion of macro 'int'
    6 | void SieveOfEratosthenes(int n, bool prime[],
      |                          ^~~
a.cc:2:13: warning: use of 'auto' in parameter declaration only available with '-std=c++20' or '-fconcepts'
    2 | #define int auto
      |             ^~~~
a.cc:7:46: note: in expansion of macro 'int'
    7 |                          bool primesquare[], int a[])
      |                                              ^~~
a.cc:2:13: warning: use of 'auto' in parameter declaration only available with '-std=c++20' or '-fconcepts'
    2 | #define int auto
      |             ^~~~
a.cc:51:19: note: in expansion of macro 'int'
   51 | int countDivisors(int n)
      |                   ^~~
a.cc: In function 'auto countDivisors(auto:30)':
a.cc:60:9: sorry, unimplemented: variable-length array of 'auto'
   60 |     int a[n]; // for storing primes upto n
      |         ^
a.cc:65:48: error: 'a' was not declared in this scope
   65 |     SieveOfEratosthenes(n, prime, primesquare, a);
      |                                                ^
a.cc: At global scope:
a.cc:2:13: error: '::main' must return 'int'
    2 | #define int auto
      |             ^~~~
a.cc:109:1: note: in expansion of macro 'int'
  109 | int main()
      | ^~~
a.cc: In function 'int main()':
a.cc:2:13: error: declaration of 'auto n' has no initializer
    2 | #define int auto
      |             ^~~~
a.cc:111:4: note: in expansion of macro 'int'
  111 |    int n; cin>>n;
      |    ^~~