c = input()
n = input()
a = map(int, raw_input().split())
dp = [c+1]*(c+1)
dp[0] = 0
ans = c+1
for e in a:
    for j in xrange(e, c+1):
        v=dp[j-e]+1
        if v<dp[j]:dp[j]=v
    if dp[c]<ans:ans=dp[c]
print-(ans>c)or ans