String Transformations - KOTLIN

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


import java.util.*

var sc = Scanner(System.`in`)

fun main() {
    val n = sc.nextInt()
    val k = sc.nextInt()
    val str = sc.next()
    println(stringTransformations(n, k, str))
}

fun stringTransformations(n: Int, k: Int, s: String): Int {
    val str = s.toCharArray()
    var first = -1
    var last = -1
    for (i in 0 until n) {
        if (str[i] == '1') {
            if (first == -1) {
                first = i
            } else {
                last = i
            }
        }
    }
    if (first == -1) {
        return 0
    } else if (last == -1) {
        return 1
    }
    var count = 2
    str[first] = '2'
    str[last] = '2'
    var prevChanged = first
    while (prevChanged < last) {
        if (last - prevChanged <= k) return count
        var i = -1
        for (j in prevChanged + 1..Math.min(n - 1, prevChanged + k)) {
            if (str[j] == '1') {
                i = j
            }
        }
        if (i == -1) {
            return -1
        }
        prevChanged = i
        str[prevChanged] = '2'
        count++
    }
    return count
}