package main

import . "fmt"
import . "os"
import bf "bufio"

func main() {
	rd:=bf.NewReader(Stdin)
	var n,m int
	Fscan(rd,&n,&m)
	x:=make([]int,n)
	for i:=range x {
		x[i]=i
	}
	t:=map[int64]bool{}
	for i:=0;i<m;i++ {
		var s int
		Fscan(rd,&s)
		a,b:=x[s-1],x[s]
		x[s-1],x[s]=b,a
		if a<b {
			a,b=b,a
		}
		t[int64(b)*int64(n+1)+int64(a)]=true
	}
	Println(len(t))
}