Help me in solving DIVIDING problem

My issue

Why cannot we use sum of first n natural number formula here

My code

#include <stdio.h>

int main(void) {
	// your code goes here
	int n;
	int count=0,sum=0;
	int arr[n];
	for(int i=0;i<n;i++){


	return 0;

Problem Link: DIVIDING Problem - CodeChef

We can use sum of first n natural number formula here.

Code :-

using namespace std;
#define int long long

int32_t main(){
    int n;
    cin >> n;
    int sum = 0;
    for(int i=0; i<n; i++){
        int temp;
        cin >> temp;
        sum += temp;
    if(sum == n*(n+1)/2.0){
        cout << "YES" << endl;
    } else {
        cout << "NO" << endl;
    return 0;
} ```

The code you’ve provided appears to be checking whether the sum of the elements in an array arr is equal to the sum of the first n natural numbers, where n is the length of the array. While you can use the formula for the sum of the first n natural numbers to optimize this code, you should note that the code as written also counts the index values (1, 2, 3, …) in the sum variable. If you want to compare only the sum of the array elements to the sum of natural numbers, you can simplify the code.