import numpy as np

C = int(input())
N = int(input())
A = np.array(list(map(int, input().split())))

MAX = C * 2
dp = np.full(C + 1, MAX)
dp[0] = 0
for i in range(C):
    dp[A[np.where(A <= C - i)] + i] = np.minimum(dp[A[np.where(A <= C - i)] + i], dp[i] + 1)
print(ans if (ans :=dp[C]) != MAX else -1)