//求助大佬，这样做为啥不对

#include <bits/stdc++.h>
using namespace std;
int a[1010];
int main()
{
	int x, n;
	scanf("%d%d", &x, &n);
	for (int i = 1; i <= n; i++)
	{
		scanf("%d", &a[i]);
	}
	sort(a + 1, a + 1 + n);
	if (a[1] != 1)
	{
		printf("-1");
		return 0;
	}
	long long ans = 0, sum = 0, idx = 1;
	while (sum < x)
	{
		ans++;
		sum += a[idx];
		if (sum >= (a[idx + 1] - 1) && idx < n) idx++;
	}
	printf("%lld", ans);
	return 0;
}