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 Java (OpenJDK 11.0.6) (judge verdict: Accepted)

import java.io.DataInputStream;
import java.io.FileInputStream;
import java.io.IOException;
import java.util.*;

public class Main {
    static class Reader {
        final private int BUFFER_SIZE = 1 << 16;
        private DataInputStream din;
        private byte[] buffer;
        private int bufferPointer, bytesRead;

        public Reader() {
            din = new DataInputStream(System.in);
            buffer = new byte[BUFFER_SIZE];
            bufferPointer = bytesRead = 0;
        }

        public Reader(String file_name) throws IOException {
            din = new DataInputStream(new FileInputStream(file_name));
            buffer = new byte[BUFFER_SIZE];
            bufferPointer = bytesRead = 0;
        }

        public String readLine() throws IOException {
            byte[] buf = new byte[64]; // line length
            int cnt = 0, c;
            while ((c = read()) != -1) {
                if (c == '\n')
                    break;
                buf[cnt++] = (byte) c;
            }
            return new String(buf, 0, cnt);
        }

        public int nextInt() throws IOException {
            int ret = 0;
            byte c = read();
            while (c <= ' ')
                c = read();
            boolean neg = (c == '-');
            if (neg)
                c = read();
            do {
                ret = ret * 10 + c - '0';
            } while ((c = read()) >= '0' && c <= '9');

            if (neg)
                return -ret;
            return ret;
        }

        public long nextLong() throws IOException {
            long ret = 0;
            byte c = read();
            while (c <= ' ')
                c = read();
            boolean neg = (c == '-');
            if (neg)
                c = read();
            do {
                ret = ret * 10 + c - '0';
            }
            while ((c = read()) >= '0' && c <= '9');
            if (neg)
                return -ret;
            return ret;
        }

        public double nextDouble() throws IOException {
            double ret = 0, div = 1;
            byte c = read();
            while (c <= ' ')
                c = read();
            boolean neg = (c == '-');
            if (neg)
                c = read();

            do {
                ret = ret * 10 + c - '0';
            }
            while ((c = read()) >= '0' && c <= '9');

            if (c == '.') {
                while ((c = read()) >= '0' && c <= '9') {
                    ret += (c - '0') / (div *= 10);
                }
            }

            if (neg)
                return -ret;
            return ret;
        }

        private void fillBuffer() throws IOException {
            bytesRead = din.read(buffer, bufferPointer = 0, BUFFER_SIZE);
            if (bytesRead == -1)
                buffer[0] = -1;
        }

        private byte read() throws IOException {
            if (bufferPointer == bytesRead)
                fillBuffer();
            return buffer[bufferPointer++];
        }

        public void close() throws IOException {
            if (din == null)
                return;
            din.close();
        }
    }
    static class Edge {
        int source;
        int destination;
        int weight;

        public Edge(int source, int destination, int weight) {
            this.source = source;
            this.destination = destination;
            this.weight = weight;
        }
    }

    static class Tree {
        int root;
        int vertices;
        ArrayList<ArrayList<Edge>> al;

        public Tree(int vertices, int root) {
            this.root = root;
            this.vertices = vertices;
            al = new ArrayList<>(vertices);
            for (int i = 0; i < vertices; i++) {
                al.add(new ArrayList<Edge>());
            }
        }

        public static void addEdgeUndirected(ArrayList<ArrayList<Edge>> al, int source, int destination, int weight) {
            al.get(source).add(new Edge(source, destination, weight));
            al.get(destination).add(new Edge(destination, source, weight));
        }

        public static void addEdgeDirected(ArrayList<ArrayList<Edge>> al, int source, int destination, int weight) {
            // this function adds an edge from vertex u to vertex v in directed graph
            al.get(source).add(new Edge(source, destination, weight));
        }

        public static void bfs(ArrayList<ArrayList<Edge>> al, int v, int root) {
            //here root means the starting vertex for bfs
            boolean[] visited = new boolean[v];
            Queue<Integer> q = new LinkedList<>();
            visited[root] = true;
            q.add(root);
            while (!q.isEmpty()) {
                int u = q.poll();
                System.out.print(u + " ");
                for (int i = 0; i < al.get(u).size(); i++) {
                    if (!visited[al.get(u).get(i).destination]) {
                        visited[al.get(u).get(i).destination] = true;
                        q.add(al.get(u).get(i).destination);
                    }
                }
            }
        }
        public static void recursiveDFS(ArrayList<ArrayList<Edge>> al, int root, boolean[] visited) {
            visited[root] = true;
            System.out.print(root + " ");
            for (int i = 0; i < al.get(root).size(); i++) {
                if (!visited[al.get(root).get(i).destination]) {
                    visited[al.get(root).get(i).destination] = true;
                    recursiveDFS(al, al.get(root).get(i).destination, visited);
                }
            }
        }

        public static void dfs(ArrayList<ArrayList<Edge>> al, int v, int root) {
            boolean[] visited = new boolean[v];
            recursiveDFS(al, root, visited);
        }
    }

    public static void main(String[] args) throws IOException {
        Reader r = new Reader();
        int n = r.nextInt();
        int m = r.nextInt();
        int sum = 0;
        for(int i=0;i<m;i++) sum += r.nextInt();
        if((n-sum)>=0) System.out.println(n-sum);
        else System.out.println(-1);
    }
}