C = int(raw_input())
N = int(raw_input())
a = map(int,raw_input().split(" "))
a.sort()
dp = {C:0}
for i in range(N):
	tmp_dp = dict()
	for c,s in dp.iteritems():
		for j in range(c/a[i],-1,-1):
			tmp_c = c - a[i]*j
			tmp_s = s + j
			if not (tmp_c in tmp_dp and tmp_dp[tmp_c] <= tmp_s):
				tmp_dp[tmp_c] = tmp_s
	dp = tmp_dp
if 0 in dp:
	print dp[0]
else:
	print -1