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.*;
import java.util.*;
import java.util.function.*;

public class Main {

	static MyReader sc = new MyReader();
	static PrintWriter out = new PrintWriter(System.out);

	public static void main(String[] args) throws IOException {
		solve();
		out.close();
	}

	static void solve() throws IOException {
		int n = sc.nextInt();
		int m = sc.nextInt();
		for(int i=0;i<m;i++){
			n -= sc.nextInt();
		}
		out.println(n>=0 ? n : -1);





	}

	//GCD
	static int GCD(int n, int m) {
		if(m > n)
			return GCD(m, n);
		if(n % m == 0)
			return m;
		else {
			return GCD(m, n % m);
		}
	}

}

class MyReader {
	private static BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
	private static String[] line = new String[0];
	private static int num = 0;

	protected MyReader() {

	}

	private void nextline() throws IOException {
		String s = in.readLine();
		line = s.split(" ");
	}

	protected String next() throws IOException {
		if(num == line.length) {
			nextline();
			num = 0;
		}
		String ret = line[num];
		num++;
		return ret;
	}

	protected int nextInt() throws IOException {
		if(num == line.length) {
			nextline();
			num = 0;
		}
		int ret = Integer.valueOf(line[num]);
		num++;
		return ret;
	}

	protected long nextLong() throws IOException {
		if(num == line.length) {
			nextline();
			num = 0;
		}
		long ret = Long.valueOf(line[num]);
		num++;
		return ret;
	}

}

// Combination
class Combination {
	public static int MAX = 510000;
	public static int MOD = 1000000007;

	public static long[] f = new long[MAX];
	public static long[] finv = new long[MAX];
	public static long[] inv = new long[MAX];

	protected Combination() {
		f[0] = f[1] = 1;
		finv[0] = finv[1] = 1;
		inv[1] = 1;
		for(int i = 2; i < MAX; i++) {
			f[i] = f[i - 1] * i % MOD;
			inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD;
			finv[i] = finv[i - 1] * inv[i] % MOD;
		}
	}

	public long Com(int n, int k) {
		if(n < k)
			return 0;
		if(n < 0 || k < 0)
			return 0;
		return f[n] * (finv[k] * finv[n - k] % MOD) % MOD;
	}

	public long Inverse(int n) {
		return inv[n];
	}

	public long Factorial(int n) {
		return f[n];
	}
}

//Tree
class Tree{
	private int n;
	private int root;
	private List<Integer>[] v;
	private int[] dist;
	private int[] size;

	protected Tree(int n){
		this.n = n;
		v = new ArrayList[n];
		for(int i=0;i<n;i++){
			v[i] = new ArrayList<>();
		}
		dist = new int[n];
		size = new int[n];
	}

	public void add(int i, int j){
		v[i].add(j);
		v[j].add(i);
	}

	public List<Integer> getV(int num){
		return v[num];
	}
	public void setV(List<Integer> tmp, int num){
		v[num] = new ArrayList<>(tmp);
	}
	public void setRoot(int num){
		root = num;
	}
	public int getRoot(){
		return root;
	}
	public int getDist(int i){
		return dist[i];
	}
	public int getSize(int i){
		return size[i];
	}

	public void distCalc(){
		size[root] = calc(root, -1, 0);
	}
	public void distCalc(int i){
		root = i;
		size[root] = calc(root, -1, 0);
	}
	private int calc(int here, int parent, int num){
		dist[here] = num;
		int s = 1;
		for(Integer e : v[here]){
			if(e!=parent) s += calc(e, here, num+1);
		}
		size[here] = s;
		return s;
	}
}


// Graph
class Graph {
	private int n;
	private List<Integer>[] v;
	private int[][] dist;

	protected Graph(int num) {
		n = num;
		v = new ArrayList[n];
		for(int i = 0; i < n; i++) {
			v[i] = new ArrayList<>();
		}
		dist = new int[n][n];
		for(int i = 0; i < n; i++) {
			for(int j = 0; j < n; j++) {
				if(i == j)
					dist[i][j] = 0;
				else
					dist[i][j] = 10000000;
			}
		}
	}

	public void add(int i, int j) {
		v[i].add(j);
		v[j].add(i);
		dist[i][j] = 1;
		dist[j][i] = 1;
	}

	public void add(int i, int j, int c){
		v[i].add(j);
		v[j].add(i);
		dist[i][j] = c;
		dist[j][i] = c;
	}

	public int BFS(int i, int j) {
		boolean[] check = new boolean[n];
		Arrays.fill(check, false);
		check[i] = true;
		Queue<Integer> queue = new ArrayDeque<>();
		queue.add(i);
		while(!queue.isEmpty()) {
			int tmp = queue.poll();
			for(Integer e : v[tmp]) {
				if(!check[e]) {
					dist[i][e] = dist[i][tmp] + 1;
					queue.add(e);
				}
			}
		}
		return dist[i][j];
	}

	public void WarshalFloyd() {
		for(int k = 0; k < n; k++) {
			for(int i = 0; i < n; i++) {
				for(int j = 0; j < n; j++) {
					dist[i][j] = Math.min(dist[i][j], dist[i][k] + dist[k][j]);
				}
			}
		}
	}

	public int getDist(int i, int j){
		return dist[i][j];
	}

}

// union-find木
class UnionFind {
	private int[] parent;
	private int[] depth;
	private int[] size;

	protected UnionFind(int n) {
		super();
		parent = new int[n];
		for(int i = 0; i < n; i++) {
			parent[i] = i;
		}
		depth = new int[n];
		size = new int[n];
		for(int i = 0; i < n; i++) {
			depth[i] = 1;
			size[i] = 1;
		}
	}

	public int Parent(int n) {
		if(parent[n] == n) {
			return n;
		} else {
			return Parent(parent[n]);
		}
	}

	public void Union(int a, int b) {
		int pa = Parent(a);
		int pb = Parent(b);
		if(pa != pb) {
			if(depth[pa] > depth[pb]) {
				parent[pb] = pa;
				size[pa] += pb;
			} else if(depth[pa] == depth[pb]) {
				parent[pb] = pa;
				size[pa] += pb;
				depth[pa]++;
			} else {
				parent[pa] = pb;
				size[pb] += pa;
			}
		}
	}

	public int Size(int n) {
		return size[Parent(n)];
	}

}