import math, sequtils, strutils
 
proc getInts(): auto = stdin.readLine.split.map parseInt
 
var k, n, f: int
(k, n, f) = getInts()
let a = getInts()
 
let
  cnt = k * n
  old = sum a
 
if cnt >= old:
  echo cnt - old
else:
  echo -1