Reduce String - KOTLIN

// Author : Saurav Kalsoor
// Reduce String - KOTLIN

import java.util.*

var sc: Scanner = Scanner(System.`in`)

fun main() {
    val n: String = sc.next()
    println(reduceString(n))
}

fun reduceString(originalString: String): String? {
    var originalString = originalString
    if (originalString.length <= 2)
        return originalString
    val newString = ""
    originalString = recurse(originalString, newString)
    return reduceString(originalString)
}

fun recurse(oldString: String, newString: String): String {
    var oldString = oldString
    var newString = newString

    if (oldString.isEmpty())
        return newString

    val n = oldString.length

    if (n == 1) {
        return newString + oldString
    }
    val first = oldString[0] - '0'
    val last = oldString[n - 1] - '0'
    val sum = first + last
    oldString = oldString.substring(1, n - 1)
    newString += Integer.toString(sum)
    return recurse(oldString, newString)
}