import java.io.*;
import java.util.*;

class prob4
{
	public static void main (String [] args) throws IOException 
	{
   
    	BufferedReader f = new BufferedReader(new FileReader("DATA4.txt"));
    	PrintWriter out = new PrintWriter(new BufferedWriter(new FileWriter("OUT4.txt")));
							
		while (f.ready())
		{
			int n = Integer.parseInt(f.readLine());
			int m = Integer.parseInt(f.readLine());
			
			int[] dp = new int[1000];
			Arrays.fill(dp, 100000);
			for (int i=0; i<m; i++)
				dp[Integer.parseInt(f.readLine())]=1;
			
			for (int i=0; i<=n; i++)
				for (int j=0; j<=n; j++)
					dp[i+j] = Math.min(dp[i+j], dp[i]+dp[j]);
			
			if (dp[n]==100000)
				out.println(-1);
			else
				out.println(dp[n]);
		}
		
		out.close();
		System.exit(0);
	}
	
	
}