Doubt in DSA learning series

How to do the ADDNTRL question in C++?
Is anything wrong in my code??
My submission is denied all the time.
#include <bits/stdc++.h>
using namespace std;

int main() {
// your code goes here
int N,a;
cin>>N;
a=(N*(N+1))/2;
cout<<a;
return 0;
}