import java.util.Arrays;
import java.util.Comparator;
import java.util.Scanner;

public class Yuki_678 {

    public Yuki_678() {
        Scanner scanner = new Scanner(System.in);
        int A = scanner.nextInt();

        int N = scanner.nextInt();
        int[] data = new int[N];
        for (int i = 0; i < N; i++) {
            data[i] = scanner.nextInt();
        }

        int[] dp = new int[100000 + 1];
        dp[0] = 0;
        for (int i = 0; i <= A; i++) {
            for (int j = 0; j < N; j++) {
                int k = i - data[j];
                if (k >= 0) {
                    dp[i] = Math.min(dp[i], dp[k] + 1);
                }
            }
        }
        if (dp[A] == 0) {
            System.out.println(-1);
        } else {
            System.out.println(dp[A]);
        }
    }

    public static void main(String[] args) {
        Yuki_678 hoge = new Yuki_678();
    }
}