# Introduction

At the Global Verification Authority, you've just been entrusted with a critical assignment.
Across the city, from online purchases to secure logins, countless operations rely on the accuracy of numerical identifiers like credit card numbers, bank account numbers, transaction codes, and tracking IDs.
The Luhn algorithm is a simple checksum formula used to ensure these numbers are valid and error-free.

A batch of identifiers has just arrived on your desk.
All of them must pass the Luhn test to ensure they're legitimate.
If any fail, they'll be flagged as invalid, preventing errors or fraud, such as incorrect transactions or unauthorized access.

Can you ensure this is done right? The integrity of many services depends on you.
