C=int(raw_input())
N=int(raw_input())
A=map(int,raw_input().split())
A.sort()
A.reverse()
dp=[10000000 for i in range(C+1)]
dp[0]=0
for a in A:
    for j in range(a,C+1):
        if dp[j-a]>=0:
            dp[j]=min(dp[j],dp[j-a]+1)
if dp[C]==10000000:
    print -1
else:
    print dp[C]