import sys
def I(): return int(sys.stdin.readline().rstrip())
def LI(): return list(map(int,sys.stdin.readline().rstrip().split()))


C,N = I(),I()
A = LI()
inf = 10**18

dp = [0]*(C+1)
# dp[i] = (sum(AjXj) == i 下での sum(Xj) の最小値)
for i in range(1,C+1):
    r = inf
    for j in range(N):
        a = A[j]
        if i >= a:
            r = min(r,1+dp[i-a])
    dp[i] = r

ans = dp[-1]
print(ans if ans != inf else -1)