# PROBLEM LINK: College Library | CodeChef

# Problem Code: College Library | CodeChef

# Practice: CodeChef | Competitive Programming | Participate & Learn | CodeChef

# Contest : Code Blooded 2.0 Coding Competition | CodeChef

* Author:* Codechef Adgitm Chapter : https://www.codechef.com/users/test_account_9

*Codechef Adgitm Chapter : https://www.codechef.com/users/test_account_9*

**Tester:***Codechef Adgitm Chapter : https://www.codechef.com/users/test_account_9*

**Editorialist:**# DIFFICULTY:

Easy

# PROBLEM:

In your college library, there are N Volumes of Novel. You are given all volume numbers between 1,2,…,n except one. Your task is to find the missing volume number.

# EXPLANATION:

By knowing this property of the natural numbers a simple solution emerges. Since we know there is only one number missing in the permutation [1, N] then we can subtract the sum of the input from the sum of all the natural numbers up to N, by doing this every number will cancel out except one, the missing number.

# SOLUTION:

C++:

#include <bits/stdc++.h>

using namespace std;

int main() {

long long int n, input, sum = 0;

cin >> n;

for (int i = 0 ; i < n - 1 ; i++) {

cin >> input;

sum += input;

}

cout << ( n * (n + 1) ) / 2 - sum << endl;

return 0;

}