Find the Number

Problem Link: Contest Page | CodeChef
Difficulty: Cake walk
Explanation;We know the sum of n natural number is 1+ 2+ … + n = n(n+1) / 2, so if we subtract the sum of all the numbers given in the question form n(n+1) / 2 then the result will give the missing number.
CODE:
#include<bits/stdc++.h>
using namespace std;
#define int long long
signed main()
{
int n;
cin >> n;
int sum = 0;
for (int i = 0; i < n - 1; i++)
{
int a;
cin >> a;
sum += a;
}
cout << (n * (n + 1) / 2) - sum;
return 0;
}