using System;
using System.IO;
using System.Collections;
using System.Collections.Generic;
using System.Text;
using System.Linq;


class Template
{
	static Scanner sc;
	public static void Main(string[] args)
	{
		const int INF = (int)1e9;
		sc = new Scanner();
		int C = sc.nextInt();
		int N = sc.nextInt();
		int[] a = sc.nextIntArray();
		Array.Sort(a);
		Array.Reverse(a);
		int[] dp = new int[C * 2];
		for (int i = 0; i < dp.Length; i++)
		{
			dp[i] = INF;
		}
		//dp[i] : f(...) = iになるxの和の最小値
		dp[0] = 0;
		//while (true)
		{
			for (int i = 0; i < C; i++)
			{
				foreach (var el in a)
				{
					if (i + el <= C && dp[i] != INF)
					{
						dp[i + el] = Math.Min(dp[i + el], dp[i] + 1);
					}
				}
			}
		}
		if (dp[C] == INF)
		{
			dp[C] = -1;
		}
		Console.WriteLine(dp[C]);
	}
}

public class Scanner
{
	public Scanner() { }
	public string next()
	{
		return Console.ReadLine();
	}
	public int nextInt()
	{
		return int.Parse(next());
	}
	public double nextDouble()
	{
		return double.Parse(next());
	}
	public long nextLong()
	{
		return long.Parse(next());
	}
	public string[] nextArray()
	{
		return next().Split(' ');
	}
	public int[] nextIntArray()
	{
		return Array.ConvertAll(nextArray(), e => int.Parse(e));
	}
	public long[] nextLongArray()
	{
		return Array.ConvertAll(nextArray(), e => long.Parse(e));
	}
	public double[] nextDoubleArray()
	{
		return Array.ConvertAll(nextArray(), e => double.Parse(e));
	}
}