**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;

}