### PROBLEM LINK:

**Author:** Konstantin Sokol

**Tester:** Gerald Agapov

**Editorialist:** Tasnim Imran Sunny

### DIFFICULTY:

Cakewalk

### PREREQUISITES:

Simple Math

### PROBLEM:

Given a list of N integers check if their sum is the same as the sum of first N natural numbers.

### EXPLANATION:

The required stamp division is possible if:

**C _{1} + … + C_{N} = 1 + 2 + … + N**

There’s a very common formula for the sum of first **N** natural numbers that is:

* 1 + 2 + … + N = N(N+1)/2**. (Proof)

So just compute the sum of all the numbers and check if the sum is N*(N+1)/2. Be careful of overflow, use 64 bit integers for computing the sum.

### AUTHOR’S AND TESTER’S SOLUTIONS:

Author’s solution can be found here.

Tester’s solution can be found here.