#!/usr/bin/env pypy3

import array
import itertools


IMPOSSIBLE = -1
INF = 10 ** 6


def compute_min(c, n, bs):
    dp = array.array("L", (INF for _ in range(c + 1)))
    dp[0] = 0
    for i, b in itertools.product(range(1, c + 1), bs):
        if b <= i:
            dp[i] = min(dp[i], dp[i - b] + 1)
    return dp[c] if dp[c] < INF else IMPOSSIBLE


def main():
    c = int(input())
    n = int(input())
    bs = array.array("L", map(int, input().split()))
    print(compute_min(c, n, bs))


if __name__ == '__main__':
    main()