type Scanner() = class
    let mutable hold = [||]
    let mutable index = 0
    member public this.Next() =
        if hold.Length > index
        then 
            index <- index + 1
            hold.[index - 1]
        else
            let mutable st = stdin.ReadLine()
            while st = "" do st <- stdin.ReadLine()
            hold <- st.Split(' ')
            if hold.Length = 0
            then
                this.Next()
            else
                index <- 1
                hold.[index - 1]
            
    member public this.NextI32() =
        this.Next() |> int

    member public this.ArrayI32(n: int) =
        [for _ in 0 .. n - 1 -> this.NextI32()]

    member public this.NextI64() =
        this.Next() |> int64
    
    member public this.ArrayI64(n: int) =
        [for _ in 0 .. n - 1 -> this.NextI64()]

    member public this.NextF32() =
        this.Next() |> float32
    
    member public this.ArrayF32(n: int) =
        [for _ in 0 .. n - 1 -> this.NextF32()]

    member public this.NextF64() =
        this.Next() |> double
    
    member public this.ArrayF64(n: int) =
        [for _ in 0 .. n - 1 -> this.NextF64()]

    member public __.Tuple2List n (f : int -> 'a list) =
        [for _ in 0 .. n - 1 -> f 2 |> fun x -> x.[0], x.[1]]
    
    member public __.Tuple3List n (f : int -> 'a list) =
        [for _ in 0 .. n - 1 -> f 3 |> fun x -> x.[0], x.[1], x.[2]]

end

[<EntryPoint>]
let main _ =
    let scanner = Scanner()
    let n = scanner.NextI32()
    let m = scanner.NextI32()
    let liA = scanner.ArrayI32 m |> List.sort
    if n >= m then 0
    else
        let a = 
            [for i in 0..m-2 -> liA.[i + 1] - liA.[i]]
            |> List.sort
            |> List.rev
        let b = List.skip (n - 1) a |> List.sum
        b
    |> printfn "%i"

    0
