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

namespace 練習問題
{
    class Program {
        static void Main(string[] args) {
            List<int> pattern = new List<int>();
            List<int> zikan = new List<int>();
            int n = 0;
            int m = 0;
            int judge = 0;
            string[] input;
            var sw = new System.Diagnostics.Stopwatch();
            debug Debug = new debug();
            if (args.Length == 0){
                Debug.FlagFalse();
                string[] nminput = Console.ReadLine().Split(' ');
                n = int.Parse(nminput[0]);
                m = int.Parse(nminput[1]);
                // c = int.Parse(Console.ReadLine());
                // string d = Console.ReadLine();
                input = Console.ReadLine().Split(' ');
            } else {
                Debug.FlagTrue();
                if (args[0] == "1"){
                    n = 41;
                    m = 2;
                    input = new string[2] {"5","6"};
                } else if (args[0] == "2"){
                    n = 10;
                    m = 2;
                    input = new string[2] {"5","6"};
                } else if (args [0] == "3"){
                    n = 11;
                    m = 2;
                    input = new string[2] {"5","6"};
                } else if (args[0] == "4"){
                    n = 314;
                    m = 15;
                    input = new string[15] {"9" ,"26" ,"5" ,"35" ,"8" ,"9" ,"79" ,"3" ,"23" ,"8","46","2","6","43","3"};
                } else {
                    n = 1;
                    m = 1;
                    input = new string[1] {"1"};
                }
            }
            sw.Start(); //2秒以内ですねスタート
            for (int i = 0; i < m; i++) {
                zikan.Add(int.Parse(input[i]));
            }
            for (int i = 0; i < zikan.Count; i++) {
                judge += zikan[i];
            }
            if (judge <= n){
                Console.WriteLine(n - judge);
            } else {
                Console.WriteLine(-1);
            }
            sw.Stop();
            TimeSpan ts = sw.Elapsed;
            // Console.WriteLine($"{ts}");
        }
        
        static int intPow(int target,int kaisu){
            int startTarget = target;
            for (int i = 1; i < kaisu; i++)
            {
                target *= startTarget;
            }
            return target;
        }

        static void bubble_sort(List<List<int>> a) {
            int count=0;
            for (int i = 0; i < a.Count; i++) {
                for (int j = 0; j < a.Count-i-1; j++) {
                    // if (a[j] == a[a.Count - 1]){
                    //     continue;
                    // }
                    count++;
                    if (a[j][1] > a[j+1][1]){
                        int localsave = 0;
                        localsave = a[j+1][1];
                        a[j+1][1] = a[j][1];
                        a[j][1] = localsave;
                        localsave = a[j][0];
                        a[j][0] = a[j+1][0];
                        a[j+1][0] = localsave;
                    }
                }
                //debug(a,"途中" + i);
            }
            // debug(a,"result");
            //Console.WriteLine("回数"+count);
            for (int i = 0; i < a.Count; i++)
            {
                Console.WriteLine(a[i][0]+" "+a[i][1]);
            }
        }
        /* 標準入力と標準出力が使えるかのチェック
        static void echo() {
            string echotest = Console.ReadLine();
            Console.WriteLine(echotest);
        }

        static void HelloWorld(){
            Console.WriteLine("Hello World!");
        }
        */
    }

    public class debug{ //宣言時は、「 debug Debug = new debug(); 」推奨
        bool debug_flag = true;
        public void Log(string str){
            if (debug_flag == true){
                Console.WriteLine(str);
            }
        }
        public void Log(int str){
            if (debug_flag == true){
                Console.WriteLine(str);
            }
        }
        public void Log(float str){
            if (debug_flag == true){
                Console.WriteLine(str);
            }
        }
        public void Log(object obj){
            if (debug_flag == true){
                Console.WriteLine(obj);
            }
        }

        public void Log(int[] arr){
            if (debug_flag == true){
                Console.WriteLine(string.Join(",", arr));
            }
        }
        public void Log(string[] arr){
            if (debug_flag == true){
                Console.WriteLine(string.Join(",", arr));
            }
        }
        public void Log(List<int> arr){
            if (debug_flag == true){
                string shuturyoku = "";
                for (int i = 0; i < arr.Count; i++) {
                    shuturyoku += arr[i] + ",";
                }
                Console.WriteLine(shuturyoku.TrimEnd(','));
            }
        }

        public void Log(List<string> arr){
            if (debug_flag == true){
                string shuturyoku = "";
                for (int i = 0; i < arr.Count; i++) {
                    shuturyoku += arr[i] + ",";
                }
                Console.WriteLine(shuturyoku.TrimEnd(','));
            }
        }

        public void Log(List<float> arr){
            if (debug_flag == true){
                string shuturyoku = "";
                for (int i = 0; i < arr.Count; i++) {
                    shuturyoku += arr[i] + ",";
                }
                Console.WriteLine(shuturyoku.TrimEnd(','));
            }
        }
        public void Log(){
            if (debug_flag == true){
                Console.WriteLine("アタイイレナサイ");
            }
        }
        public void FlagFalse(){
            debug_flag = false;
        }
        public void FlagTrue(){
            debug_flag = true;
        }
    }
}

Measured memory_bytes: 28708