def dfs(C,S,idx):
	if idx == N:
		if C == 0:
			return S
		else:
			return None
	ret = None
	for i in range(C/a[idx],-1,-1):
		ret = dfs(C-a[idx]*i,S+i,idx+1)
		if ret is not None:
			break
	return ret

C = int(raw_input())
N = int(raw_input())
a = map(int,raw_input().split(" "))
a.sort(reverse = True)
ans = dfs(C,0,0)
if ans is None:
	print -1
else:
	print ans