guys , please tell what is the error in this program as it is not printing anything on screen as output

This code is for finding the missing number in array but after running this code no output is visible what is wrong in this code ?

#include<bits/stdc++.h>
using namespace std;
int main()
{
  long long int n , a[n] , sum ;
  cin>>n;
  for (int i = 0; i < n; ++i)
{
    cin >> a[i];

}

 sum = n*(n+1)/2;
// cout<<sum;
for(int i=0; i<n;i++)
{
  sum=sum-a[i];
}
cout << sum;
//return 0;
}