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[] arr = new int[n];
        for (int i = 0; i < n; i++) {
            arr[i] = sc.nextInt();
        }
        Arrays.sort(arr);
        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 && i - arr[j] >= 0; j++) {
                if (dp[i - arr[j]] != Integer.MAX_VALUE) {
                    dp[i] = Math.min(dp[i], dp[i - arr[j]] + 1);
                }
            }
        }
        if (dp[c] == Integer.MAX_VALUE) {
            System.out.println(-1);
        } else {
            System.out.println(dp[c]);
        }
    }
}