fun readInt () =
    valOf (TextIO.scanStream (Int.scan StringCvt.DEC) TextIO.stdIn)

fun int_string n =
    if 0 <= n
    then Int.toString n
    else "-" ^ Int.toString (abs n)

val () =
    let
        val k = readInt ()
        val n = readInt ()
        val f = readInt ()
        val a_s = List.tabulate (f, fn _ => readInt ())
        val sum = foldl (fn (x, acc) => x + acc) 0 a_s
        val ans = if sum <= k * n then k * n - sum else ~1
    in
        print (int_string ans)
    end