Problem:
Score : 200 points

Problem Statement

Takahashi has N days of summer vacation.

His teacher gave him M summer assignments. It will take A_i days for him to do the i-th assignment.

He cannot do multiple assignments on the same day, or hang out on a day he does an assignment.

What is the maximum number of days Takahashi can hang out during the vacation if he finishes all the assignments during this vacation?

If Takahashi cannot finish all the assignments during the vacation, print -1 instead.

Constraints

1 \leq N \leq 10^6

1 \leq M \leq 10^4

1 \leq A_i \leq 10^4

Input

Input is given from Standard Input in the following format:

N M
A_1 ... A_M

Output

Print the maximum number of days Takahashi can hang out during the vacation, or -1.

Sample Input 1

41 2
5 6

Sample Output 1

30

For example, he can do the first assignment on the first 5 days, hang out on the next 30 days, and do the second assignment on the last 6 days of the vacation. In this way, he can safely spend 30 days hanging out.

Sample Input 2

10 2
5 6

Sample Output 2

-1

He cannot finish his assignments.

Sample Input 3

11 2
5 6

Sample Output 3

0

He can finish his assignments, but he will have no time to hang out.

Sample Input 4

314 15
9 26 5 35 8 9 79 3 23 8 46 2 6 43 3

Sample Output 4

9

Solution in C# (.NET Core 3.1.201) (judge verdict: Accepted)

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.IO;
using System.Text.RegularExpressions;

namespace AtCoder2
{
    public class MainClass
    {
        public static void Main(string[] args)
        {
            var sw = new StreamWriter(Console.OpenStandardOutput(), Console.Out.Encoding);
            sw.AutoFlush = false;
            Console.SetOut(sw);

            if (args.Length > 0 && args[0] == "debug")
            {
                var inputs = new[] { @"
1
				", @"
73
                ", @"

				", @"

				", @"

				" };

                foreach (var str in inputs)
                {
                    var text = str.Trim();
                    if (text.Length == 0) continue;
                    var reader = new System.IO.StringReader(text);
                    Console.SetIn(reader);
                    Main2(args);
                    Console.WriteLine("----------------------------------");
                }
            }
            else
                Main2(args);

            Console.Out.Flush();
        }

        public static void Main2(string[] args)
        {
            checked
            {
                var s=Console.ReadLine().Split();
                var N = int.Parse(s[0]);
                var M = int.Parse(s[1]);
                var A=Console.ReadLine().Split().Select(int.Parse).ToList();
                var As = A.Sum();

                if(N<As)
                    Console.WriteLine("-1");
                else
                    Console.WriteLine(N-As);
            }
        }

        private static void Assert(bool cond)
        {
            if (!cond) throw new Exception();
        }
    }

    public static class EnumerablePlus
    {
        ///<summary>出力文字列に変換。</summary>
        public static string MkString<T>(this IEnumerable<T> list)
        {
            return list.Aggregate(
                new System.Text.StringBuilder(),
                (a, b) => ((a.Length > 0) ? a.Append(' ') : a).Append(b)
                ).ToString();
        }

        ///<summary>添字付きfor。</summary>
        public static void ForEach2<T>(this IEnumerable<T> list, Action<T, long> a)
        {
            long i = 0;
            foreach (var item in list)
            {
                a(item, i);
            }
        }

        ///<summary>添字付きfor。</summary>
        public static void ForEach<T>(this IEnumerable<T> list, Action<T> a)
        {
            foreach (var item in list)
            {
                a(item);
            }
        }

        ///<summary>指定回数デリゲートを呼び出す。</summary>
        public static IEnumerable<T> Repeat<T>(long n, Func<T> f)
        {
            for (long i = 0; i < n; i++)
            {
                yield return f();
            }
        }

        public static T MinBy<T, U>(this IEnumerable<T> xs, Func<T, U> key) where U : IComparable<U>
        {
            return xs.Aggregate((a, b) => key(a).CompareTo(key(b)) <= 0 ? a : b);
        }

        public static T MaxBy<T, U>(this IEnumerable<T> xs, Func<T, U> key) where U : IComparable<U>
        {
            return xs.Aggregate((a, b) => key(a).CompareTo(key(b)) >= 0 ? a : b);
        }

        public static List<T> MaxBy2<T, U>(this IEnumerable<T> xs, Func<T, U> key) where U : IComparable<U>
        {
            return xs.Aggregate(
                new List<T>(),
                (list, b) =>
                {
                    if (list.Count == 0) { list.Add(b); return list; }

                    var aa = key(list[0]);
                    var bb = key(b);

                    if (aa.CompareTo(bb) < 0) list.Clear();

                    if (aa.CompareTo(bb) <= 0) { list.Add(b); return list; }

                    return list;
                });
        }

        public static List<T> Max2<T>(this IEnumerable<T> xs) where T : IComparable<T>
        {
            return xs.MaxBy2(o => o);
        }

        public static void Move<T>(this IList<T> list, int from, int to)
        {
            var item = list[from];

            for (var idx = from; idx > to; --idx)
            {
                list[idx] = list[idx - 1];
            }

            list[to] = item;
        }

        public static void Add2<T>(this IDictionary<T, int> xs, T key)
        {
            if (xs.ContainsKey(key))
                xs[key]++;
            else
                xs.Add(key, 1);
        }
    }

    public static class MathPlus
    {
        ///<summary>約数。順番は不定。</summary>
        public static List<long> Divisor(long n)
        {
            var list = Prime(n).
                GroupBy(id => id).
                Select(pair => new { prime = pair.Key, power = pair.Count() }).
                ToList();

            var result = list.Aggregate(new List<long>(), (left, pair) =>
            {
                var list2 = Enumerable.
                    Range(0, pair.power + 1).
                    Aggregate(new List<long>(), (res, l) =>
                    {
                        var next = res.Count > 0 ? res.Last() * pair.prime : 1L;
                        res.Add(next);
                        return res;
                    });

                if (left.Count == 0) return list2.ToList();

                return list2.
                    SelectMany(l => left.Select(m => l * m)).
                    ToList();
            });

            return result;
        }

        ///<summary>順列の数。</summary>
        public static long Permutation(long n, long r)
            => Factorial(n, r);

        ///<summary>組合せの数。</summary>
        public static long Combination(long n, long r)
        {
            if (r == 0) return 1;

            return Factorial(n, r) / Factorial(r);
        }

        ///<summary>階乗。カウントも指定可能。</summary>
        public static long Factorial(long a, long count = long.MaxValue)
        {
            long res = 1;

            for (long i = 0; a > 0 && i < count; i++)
            {
                checked { res *= a; }
                --a;
            }

            return res;
        }

        ///<summary>素因数分解。昇順。</summary>
        public static List<long> Prime(long a)
        {
            var res = new List<long>();

            var a2 = Math.Ceiling(Math.Sqrt(a));
            for (long b = 2; b <= a2; ++b)
            {
                while (a % b == 0)
                {
                    res.Add(b);
                    a = a / b;
                    a2 = Math.Ceiling(Math.Sqrt(a));
                }
            }

            if (a > 1) res.Add(a);
            return res;
        }

        ///<summary>最小公倍数。</summary>
        public static long Lcm(IEnumerable<long> list)
        {
            return list.Aggregate((a, b) => Lcm(a, b));
        }

        ///<summary>最小公倍数。</summary>
        public static long Lcm(long a, long b)
        {
            return checked(a * b) / Gcd(a, b);
        }

        ///<summary>最大公約数。</summary>
        public static long Gcd(IEnumerable<long> list)
        {
            return list.Aggregate((a, b) => Gcd(a, b));
        }

        ///<summary>最大公約数。</summary>
        public static long Gcd(long a, long b)
        {
            if (a < b)
                Gcd(b, a);

            while (b != 0)
            {
                var rem = a % b;
                a = b;
                b = rem;
            }

            return a;
        }

        ///<summary>切り上げ。</summary>
        public static long Ceiling(long a, long b) => (a + (b - 1)) / b;
    }
}