def func(a, m, n):
    cur, count, i = 0, 0, 0
    if a[0] != 1:
        return -1
    else:
        while cur < m:
            while i + 1 < n and a[i + 1] <= cur + 1:
                i += 1
            cur += a[i]
            count += 1
        return count
m, n = map(int, input().split())
a = []
for i in range(n):
    a.append(int(input()))
a.sort()
num = func(a, m, n)
print(num)