import java.lang.Math
fun main( args : Array<String> ) {
  val (n, m) = readLine()!!.split(" ").map { it.toInt() }
  var ps = (1..m).map {
    readLine()!!.toInt()
  }.toList()
  var status = 0
  var miles  = 0
  (0..n-1).map {
    val f = ps.map { x -> 
                       Pair(x, Math.abs(x - status) ) 
                    }.sortedBy { x->
                      x.second 
                    }.first()
    miles   += f.second
    status   = f.first
    ps = ps.filter { x -> x != status }
  }
  println( miles )
}