import sys
input = lambda :sys.stdin.readline()[:-1]
ni = lambda :int(input())
na = lambda :list(map(int,input().split()))
sys.setrecursionlimit(10**7)
yes = lambda :print("yes");Yes = lambda :print("Yes");YES = lambda : print("YES")
no = lambda :print("no");No = lambda :print("No");NO = lambda : print("NO")
#######################################################################

c = ni()
n = ni()
a = na()
a = sorted(set(a))
n = len(a)
from collections import deque
d = deque([0])
inf = 10**9
dp = [inf for i in range(c+1)]
dp[0] = 0
while d:
    e = d.popleft()
    for i in a:
        if i+e>c:
            break
        if dp[i+e]!=inf:continue
        dp[i+e] = dp[e]+1
        d.append(i+e)
if dp[-1]==inf:
    print(-1)
else:
    print(dp[-1])