using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;

namespace cysharp_test
{
    class Program
    {
        static void Main (string[] args)
        {
            new Program ().solve ();
        }

        void solve ()
        {
            int[] NM = Console.ReadLine().Split(' ').Select(int.Parse).ToArray();

            int N = NM[0];
            int M = NM[1];

            int[] A = Console.ReadLine().Split(' ').Select(int.Parse).ToArray();

            if(N - A.Sum() >= 0)Console.WriteLine(N - A.Sum());
            else Console.WriteLine("-1");
        }

    }

    class Vector2
    {
        public Vector2 (long x, long y)
        {
            this.x = x;
            this.y = y;
        }

        long x, y;

        public static Vector2 operator + (Vector2 a, Vector2 b)
        {
            return new Vector2 (a.x + b.x, a.y + b.y);
        }

        public static Vector2 operator - (Vector2 a, Vector2 b)
        {
            return new Vector2 (a.x - b.x, a.y - b.y);
        }

        public static Vector2 operator * (Vector2 a, long scala)
        {
            return new Vector2 (a.x * scala, a.y + scala);
        }
    }
}