import java.util.*;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int C = sc.nextInt();
		int N = sc.nextInt();
		int[] a = new int[N];
		for(int i=0;i<N;i++)a[i]=sc.nextInt();
		int[] dp = new int[C+1];
		Arrays.fill(dp, Integer.MAX_VALUE);
		dp[0]=0;
		for(int i=1;i<=C;i++){
			for(int j=0;j<N;j++){
				if(i-a[j]<0 || dp[i-a[j]]==Integer.MAX_VALUE) continue;
				dp[i]=Math.min(dp[i],dp[i-a[j]]+1);
			}
		}
		if(dp[C]==Integer.MAX_VALUE){
			System.out.println(-1);
		}else{
			System.out.println(dp[C]);
		}
	}
}