import scala.io.{StdIn => in}
import scala.collection.{mutable => mu}

object Main extends App {
  val Array(n,k) = in.readLine.split(" ").map(_.toInt)
  val s = in.readLine.toArray.map(_ - '0')
  val buf0 = mu.ArrayBuffer.empty[Int]
  val buf1 = mu.ArrayBuffer.empty[Int]
  var cnt0 = 0
  var cnt1 = 0
  for(i <- 0 until n) {
    if(s(i) == 0) cnt0 += 1
    else if(s(i) == 1 && cnt0 > 0) {
      buf0.append(cnt0)
      cnt0 = 0
    }
    if(s(i) == 1) cnt1 += 1
    else if(s(i) == 0 && cnt1 > 0) {
      buf1.append(cnt1)
      cnt1 = 0
    }
  }
  val ans =
    if(s forall(_ == 1)) s.sum
    else
      buf1.sorted(Ordering[Int].reverse).take(k).sum +
      buf0.sorted(Ordering[Int].reverse).take(k).sum
  println(ans)
}