Sum of Digits in Kotlin (Beginner-Friendly)

fun sumOfDigits(n: Int): Int {
    require(n >= 0) { "Input must be a non-negative integer." }
    return n.toString().map { it - '0' }.sum()
}

fun main() {
    println("Enter a non-negative integer:")

    val input = readLine()
    if (input.isNullOrBlank()) {
        println("Error: No input provided.")
        return
    }

    val number = input.toIntOrNull()
    if (number == null) {
        println("Error: Input is not a valid integer.")
        return
    }

    if (number < 0) {
        println("Error: Please enter a non-negative integer.")
        return
    }

    val result = sumOfDigits(number)
    println("Sum of digits: $result")
}

/*
Sample Output:
Enter a non-negative integer:
12345
Sum of digits: 15
*/