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):
        dp[j] = min(dp[j], dp[j-e]+1)
    ans = min(ans, dp[c])
print-(ans>c)or ans