import math._

object Main {
  def calc(n: Int, xs: Array[Int]): Int = {
    var p = 1
    for (x <- xs.reverse) {
      if (p == 1) p = x
      else if (x >= p) p -= 1
    }
    p
  }

  def main(args: Array[String]) {
    val sc = new java.util.Scanner(System.in)
    val n, m = sc.nextInt
    var xs = Array.fill(m)(sc.nextInt)

    println(calc(n, xs))
  }
}