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

namespace Contest
{
    class Program
    {
        static void Main(string[] args)
        {
            ABC162.B();
        }
        class ABC162
        {
            public static void A()
            {
                var R = Read.Int();
                Console.WriteLine(R * 2 * Math.PI);
            }
            public static void B()
            {
                var N = Read.Int();
                var M = Read.Int();
                var A = Read.IntN();
                var sum = A.Sum();
                Console.WriteLine(N < sum ? -1 : N - sum);
            }
            public static void C()
            {
                var N = Read.Int();
                var A = Read.IntN();
                var count = new int[N];
                for(int i = 0; i < N - 1; ++i)
                {
                    count[A[i] - 1]++;
                }
                var sb = new System.Text.StringBuilder();
                for(int i = 0; i < N; ++i)
                {
                    sb.AppendLine(count[i].ToString());
                }
                Console.Write(sb.ToString());
            }
            public static void D()
            {
                var N = Read.Int();
                var K = Read.Int();
                long total = 0;
                var min = Enumerable.Range(0, K).Select(i => (long)i).Sum();
                var max = Enumerable.Range(0, K).Select(i => (long)N - i).Sum();
                for (int i = K; i <= N + 1; ++i)
                {
                    total = (total + max - min + 1) % Util.P;
                    min += i;
                    max += N - i;
                }
                Console.WriteLine(total);
            }
            public static void E()
            {
                var N = Read.Int();
                var A = Read.LongN();
                long max = 0;
                for(int i = 0; i <= N; ++i)
                {
                    long total = 0;
                    var queueL = new Util.PriorityQueue<Tuple<long, int>>(N, Comparer<Tuple<long, int>>.Create((t1, t2) => -t1.Item1.CompareTo(t2.Item1)));
                    for(int j = 0; j < i; ++j)
                    {
                        queueL.Push(new Tuple<long, int>(A[j], j));
                    }
                    var posL = N - 1;
                    while (queueL.Count() > 0)
                    {
                        var t = queueL.Pop();
                        total += Math.Abs(posL - t.Item2) * t.Item1;
                        posL--;
                    }
                    var queueR = new Util.PriorityQueue<Tuple<long, int>>(N, Comparer<Tuple<long, int>>.Create((t1, t2) => -t1.Item1.CompareTo(t2.Item1)));
                    for (int j = i; j < N; ++j)
                    {
                        queueL.Push(new Tuple<long, int>(A[j], j));
                    }
                    var posR = 0;
                    while (queueL.Count() > 0)
                    {
                        var t = queueL.Pop();
                        total += Math.Abs(posR - t.Item2) * t.Item1;
                        posR++;
                    }
                    max = Math.Max(max, total);
                }
                Console.WriteLine(max);
            }
            public static void F()
            {

            }
        }



        class Util
        {
            public static long P = 1000000007;
            public class PriorityQueue<T> where T : IComparable
            {
                private IComparer<T> _comparer = null; private bool _isAsc = true; private T[] _heap; private int _sz = 0;
                public PriorityQueue(int maxSize, IComparer<T> comparer) { _heap = new T[maxSize]; _comparer = comparer; }
                public PriorityQueue(int maxSize, bool isAsc = true) { _heap = new T[maxSize]; _isAsc = isAsc; }
                private int Compare(T x, T y) { if (_comparer != null) { return _comparer.Compare(x, y); } return _isAsc ? x.CompareTo(y) : y.CompareTo(x); }
                public void Push(T x) { var i = _sz++; while (i > 0) { var p = (i - 1) / 2; if (Compare(_heap[p], x) <= 0) { break; } _heap[i] = _heap[p]; i = p; } _heap[i] = x; }
                public T Pop() { T ret = _heap[0]; T x = _heap[--_sz]; int i = 0; while (i * 2 + 1 < _sz) { int a = i * 2 + 1; int b = i * 2 + 2; if (b < _sz && Compare(_heap[b], _heap[a]) < 0) { a = b; } if (Compare(_heap[a], x) >= 0) { break; } _heap[i] = _heap[a]; i = a; } _heap[i] = x; return ret; }
                public int Count() { return _sz; }
                public T Peek() { return _heap[0]; }
                public bool Contains(T x) { for (int i = 0; i < _sz; i++) if (x.Equals(_heap[i])) return true; return false; }
            }

            public static long Power(long a, long n, long p)
            {
                long res = 1;
                long mult = a;
                for (int i = 0; i < 40; ++i)
                {
                    if ((n & ((long)1 << i)) > 0)
                    {
                        res = (res * mult) % p;
                    }
                    mult = (mult * mult) % p;
                }
                return res;
            }
            public static long Inverse(long a, long p)
            {
                long b = p, c = 1, d = 0, e = 0, f = 0;
                while (b != 0)
                {
                    e = a / b;
                    f = b; b = a - e * b; a = f;
                    f = d; d = c - e * d; c = f;
                }
                c %= p;
                return c + (c < 0 ? p : 0);
            }

            public static long[] Facts(int n, long p)
            {
                var facts = new long[n + 1];
                facts[0] = 1;
                for (int i = 1; i <= n; ++i)
                {
                    facts[i] = (i * facts[i - 1]) % p;
                }
                return facts;
            }
            public static Func<int, int, long> Combin(int n, long P)
            {
                var fact = Facts(n, P);
                var inv = fact.Select(x => Inverse(x, P)).ToArray();
                return (N, M) =>
                {
                    if (N < 0 || N < M || M < 0) { return 0; }
                    return (((fact[N] * inv[M]) % P) * inv[N - M]) % P;
                };
            }

            public static HashSet<long> Divisor(long x)
            {
                var div = new HashSet<long>(); for (long p = 1; p * p <= x; ++p) { if (x % p == 0) { div.Add(p); } }
                foreach (var d in div.ToArray())
                {
                    if (!div.Contains(x / d)) { div.Add(x / d); }
                }
                return div;
            }
            public class BIT
            {
                int N; long[] array;
                public BIT(int m) { for (int n = 1; ; n *= 2) { if (n >= m) { N = n; array = new long[N + 1]; break; } } }
                public void Add(int idx, int v) { for (int x = idx; x <= N; x += x & -x) { array[x] += v; } }
                public long Sum(int idx) { long ret = 0; for (int x = idx; x > 0; x -= x & -x) { ret += array[x]; } return ret; }
            }
        }
        struct MOD
        {
            public static long P = 1000000007;
            public readonly long V;

            public MOD(long a) { this.V = a; }
            public static implicit operator MOD(long a) { return new MOD(a); }
            public static MOD operator +(MOD a, MOD b) { return new MOD((a.V + b.V) % P); }
            public static MOD operator -(MOD a, MOD b) { return new MOD((a.V - b.V + P) % P); }
            public static MOD operator *(MOD a, MOD b) { return new MOD((a.V * b.V) % P); }
            public static MOD operator /(MOD a, MOD b) { return new MOD((a.V * b.Inverse().V) % P); }
            //public MOD Inverse() { return Util.Inverse(V, P); }
            public MOD Inverse()
            {
                long a = V, p = P;
                long b = p, c = 1, d = 0, e = 0, f = 0;
                while (b != 0)
                {
                    e = a / b;
                    f = b; b = a - e * b; a = f;
                    f = d; d = c - e * d; c = f;
                }
                c %= p;
                return c + (c < 0 ? p : 0);
            }
            //public MOD Power(long n) { return Util.Power(V, n, P); }
            public MOD Power(long n)
            {
                long a = V, p = P;
                long res = 1;
                long mult = a;
                for (int i = 0; i < 40; ++i)
                {
                    if ((n & ((long)1 << i)) > 0)
                    {
                        res = (res * mult) % p;
                    }
                    mult = (mult * mult) % p;
                }
                return res;
            }

            public override string ToString() { return V.ToString(); }
        }


        static class Read
        {
            static Queue<string> queue = new Queue<string>();
            static void Enqueue() { var ss = Console.ReadLine().Split(' '); foreach (var s in ss) { queue.Enqueue(s); } }
            public static int Int() { if (queue.Count == 0) { Enqueue(); } return Convert.ToInt32(queue.Dequeue()); }
            public static long Long() { if (queue.Count == 0) { Enqueue(); } return Convert.ToInt64(queue.Dequeue()); }
            public static string Str() { if (queue.Count == 0) { Enqueue(); } return queue.Dequeue(); }
            public static int[] IntN() { return Console.ReadLine().Split(' ').Select(s => Convert.ToInt32(s)).ToArray(); }
            public static long[] LongN() { return Console.ReadLine().Split(' ').Select(s => Convert.ToInt64(s)).ToArray(); }
            public static string[] StrN() { return Console.ReadLine().Split(' '); }
            public static Tuple<int, int>[] TupleInt(int n) { List<Tuple<int, int>> list = new List<Tuple<int, int>>(); for (int i = 0; i < n; ++i) { var ab = Read.IntN(); list.Add(new Tuple<int, int>(ab[0], ab[1])); } return list.ToArray(); }
            public static Tuple<long, long>[] TupleLong(int n) { List<Tuple<long, long>> list = new List<Tuple<long, long>>(); for (int i = 0; i < n; ++i) { var ab = Read.LongN(); list.Add(new Tuple<long, long>(ab[0], ab[1])); } return list.ToArray(); }
            public static Tuple<string, string>[] TupleStr(int n) { List<Tuple<string, string>> list = new List<Tuple<string, string>>(); for (int i = 0; i < n; ++i) { var ab = Read.StrN(); list.Add(new Tuple<string, string>(ab[0], ab[1])); } return list.ToArray(); }
            public static Tuple<double, double>[] TupleDouble(int n) { List<Tuple<double, double>> list = new List<Tuple<double, double>>(); for (int i = 0; i < n; ++i) { var ab = Read.LongN(); list.Add(new Tuple<double, double>(ab[0], ab[1])); } return list.ToArray(); }
            public static long[,] LongMatrix(int r, int c)
            {
                var mat = new long[r, c];
                for (int i = 0; i < r; ++i)
                {
                    var x = Read.LongN();
                    for (int j = 0; j < c; ++j)
                    {
                        mat[i, j] = x[j];
                    }
                }
                return mat;
            }
            public static Dictionary<int, HashSet<int>> AdjacencyList(int N, int M, bool isDirectional = false)
            {
                Dictionary<int, HashSet<int>> dict = new Dictionary<int, HashSet<int>>();
                for (int i = 0; i <= N; ++i) { dict.Add(i, new HashSet<int>()); }
                for (int i = 0; i < M; ++i)
                {
                    var ab = Read.IntN(); dict[ab[0]].Add(ab[1]);
                    if (!isDirectional) { dict[ab[1]].Add(ab[0]); }
                }
                return dict;
            }
            public static Dictionary<int, Dictionary<int, long>> AdjacencyListWithWeight(int N, int M)
            {
                Dictionary<int, Dictionary<int, long>> dict = new Dictionary<int, Dictionary<int, long>>();
                for (int i = 0; i <= N; ++i)
                {
                    dict.Add(i, new Dictionary<int, long>());
                }
                for (int i = 0; i < M; ++i)
                {
                    var ab = Read.IntN();
                    dict[ab[0]].Add(ab[1], ab[2]);
                    dict[ab[1]].Add(ab[0], ab[2]);
                }
                return dict;
            }
        }
    }
}


Measured memory_bytes: 20476