import strutils, sequtils

const inf = 987654321

var
  c = stdin.readLine.parseInt
  n = stdin.readLine.parseInt
  a = newSeqWith(n, 0)
  dp = newSeqWith(n+1, newSeqWith(c+1, -1))

var sp = stdin.readLine.split
for i in 0.. <n:
  a[i] = sp[i].parseInt

proc nya(i, j: int): int =
  if dp[i][j] != -1:
    return dp[i][j]
  if i == n:
    dp[i][j] = if c==j: 0 else: inf
    return dp[i][j]
  result = inf
  var
    p = j
    x = 0
  while p <= c:
    result = min(result, nya(i+1, p) + x)
    p += a[i]
    x += 1
  dp[i][j] = result

var res = nya(0, 0)
if res == inf:
  res = -1
echo res
# (ΦωΦ)＜にゃー