Update README.md

Author: silven-mohan

README.md

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-# Luhn_Algorithm
+# Luhn_Algorithm
+This C program validates a 16-digit number using the Luhn algorithm, a simple checksum formula used to validate identification numbers such as credit card or debit card numbers.
+
+## ✅ Features
+Validates input length (must be exactly 16 digits)
+
+Ensures all characters are numeric
+
+Implements Luhn's checksum logic:
+
+Doubles every second digit from the left (starting at index 0)
+
+Subtracts 9 from any result greater than 9
+
+Sums the remaining digits
+
+Prints whether the number is valid or invalid as per Luhn’s algorithm
+
+## 📌 How It Works
+Input: The user is prompted to enter a 16-digit number.
+
+Validation:
+
+Checks if the number has exactly 16 digits
+
+Rejects if any non-numeric character is present
+
+Processing:
+
+Doubles digits at even indices (0, 2, 4, ..., 14), adjusts if >9
+
+Adds digits at odd indices (1, 3, ..., 15) directly
+
+Result:
+
+If total sum is divisible by 10, the number is considered valid
+
+Otherwise, it is invalid.
+
+----
+
+## 🛠 Example
+```c
+Enter the number: 4539578763621486
+4539578763621486 passes Luhn's Algorithm.
+```
+
+## 📂 Function Definition
+```c
+int Luhn_Algorithm();
+```
+
+## 📚 Dependencies
+<stdio.h>
+
+<string.h>
+
+<ctype.h>
+
+## 🧠 Why Use This?
+This is a useful utility for learning how credit card validation works under the hood, practicing digit manipulation, and understanding input validation techniques in C.
+
+----