C = int(raw_input())
N = int(raw_input())
a = sorted(list(set(map(int,raw_input().split(" ")))))
M = len(a)
UB = 10**5+1
dp = [UB for i in range(C+1)]
dp[0] = 0
for i in range(1,C+1):
	for j in range(M):
		if a[j] > i: 
			break
		if dp[i-a[j]] + 1 < dp[i]: 
			dp[i] = dp[i-a[j]] + 1
			
print -1 if dp[C] == UB else dp[C]