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.util.*;
import java.io.*;

	
public class Main {
	public static void main(String[] args) throws Exception {
		FastScanner sc = new FastScanner();
		PrintWriter out = new PrintWriter(System.out);
		
		int d = sc.nextInt();
		int m = sc.nextInt();
		
		int sum=0;
    	for(int i=0; i<m; i++){
    	    sum += sc.nextInt();
    	}
    	if(sum>d) out.println(-1);
    	else out.println(d-sum);
		

	//	int[] as = new int[wt];
		
	/*for(int i=0; i<wt; i++){
		as[i] = sc.nextInt();
		}*/
		
	// 	String s =sc.next();
	// 	long nu = sc.nextLong();
		
	// 	int max=0 ,tmp=0;
	/* 	for(int i=0; i<wt; i++){
		
		if(tmp>max) max=tmp;
		}*/
		//out.println(max);
		
		out.flush();
	}


	private static int INF = (int)1e8;
	private static long INFL = (long)1e17;
	private static int e97 = (int)1e9 + 7;
	
	
	private static int abs(int a){
	return (a>=0) ? a: -a;
	}
	private static long abs(long a){
	return (a>=0) ? a: -a;
	}
	
	private static int min(int a, int b){
	return (a>b) ? b : a;
	}
	private static long min(long a, long b){
	return (a>b) ? b : a;
	}
	
	private static int max(int a, int b){
		return (a>b) ? a : b;
	}
	private static long max(long a, long b){
		return (a>b) ? a : b;
	}
	
	private static int minN(int... ins){
		int min = ins[0];
		for(int i=1; i<ins.length; i++){
			if(ins[i] < min) min = ins[i];
		}
		return min;
	}
	private static int maxN(int... ins){
		int max = ins[0];
		for(int i=1; i<ins.length; i++){
			if(ins[i] > max) max = ins[i];
		}
		return max;
	}
	private static long minN(long... ins){
		long min = ins[0];
		for(int i=1; i<ins.length; i++){
			if(ins[i] < min) min = ins[i];
		}
		return min;
	}
	private static long maxN(long... ins){
		long max = ins[0];
		for(int i=1; i<ins.length; i++){
			if(ins[i] > max) max = ins[i];
		}
		return max;
	}
	
	private static int minExAd(int[] dt, int ad){
	int min=Integer.MAX_VALUE;
		for(int i=0; i<dt.length; i++){
			if((i != ad) && (dt[i] < min)) min = dt[i];
		}
		return min;
	}
	private static long minExAd(long[] dt, int ad){
	long min=Long.MAX_VALUE;
		for(int i=0; i<dt.length; i++){
			if((i != ad) && (dt[i] < min)) min = dt[i];
		}
		return min;
	}
	private static int minExVal(int[] dt, int ex_val){
	int min=Integer.MAX_VALUE;
		for(int i=0; i<dt.length; i++){
			if((dt[i] != ex_val) && (dt[i] < min)) min = dt[i];
		}
		return min;
	}
	private static long minExVal(long[] dt, long ex_val){
	long min=Long.MAX_VALUE;
		for(int i=0; i<dt.length; i++){
			if((dt[i] != ex_val) && (dt[i] < min)) min = dt[i];
		}
		return min;
	}
	
	private static boolean same3(long a, long b, long c){
	if(a!=b) return false;
	if(b!=c) return false;
	if(c!=a) return false;
	return true;
	}
	private static boolean dif3(long a, long b, long c){
	if(a==b) return false;
	if(b==c) return false;
	if(c==a) return false;
	return true;
	}
	
	private static int factorial(int n) {
		int ans=1;
		for(int i=n; i>0; i--){
			ans*=i;
		}
		return ans;
	}
	
	private static long gcd(long m, long n) {
		if(m < n) return gcd(n, m);
		if(n == 0) return m;
		return gcd(n, m % n);
	}
	
	private static int pow2(int in){
		return in*in;
	}
	private static long pow2(long in){
		return in*in;
	}
	
	private static int getDigit2(long num){
		long cf = 1;
		int d=0;
		while(num >= cf){
			d++;
			cf = 1<<d;
		}
		return d; //numはd桁の数で、2^dより小さい
	}
	
	private static int getDigit10L(long num){
		long cf = 1;
		int d=0;
		while(num >= cf){
			d++;
			cf*=10;
		}
		return d; //numはd桁の数で、10^dより小さい
	}
	
	private static boolean isINF(int in){
	if((in*20)>INF) return true;
	else return false;
	}
	private static boolean isINFL(long in){
	if((in*10000)>INFL) return true;
	else return false;
	}
	
	private static long pow10E97(long ob, int soeji){
		if(soeji==2) return (ob*ob)%e97;
		
		int d = getDigit2(soeji);
		long[] ob_pow_2pow = new long[d];
		
		ob_pow_2pow[0] = ob;
		for(int i=1; i<d; i++){
			ob_pow_2pow[i] = (ob_pow_2pow[i-1]*ob_pow_2pow[i-1])%e97;
		}
		
		long ans=1;
		for(int i=d-1; i>=0; i--){
			if(soeji >= (1<<i)){
				soeji -= (1<<i);
				ans = (ans*ob_pow_2pow[i])%e97;
			}
 		}
		
		return ans;
	}
	
	private static int flag(int pos){
	return (1<<pos);
	}
	private static boolean isFlaged(int bit, int pos){
	if((bit&(1<<pos)) > 0) return true;
	else return false;
	}
	private static int deflag(int bit, int pos){
	return bit&~(1<<pos);
	}
	private static void showflag(int bit){
	for(int i=0; i<getDigit2(bit); i++){
	if(isFlaged(bit,i)) System.out.print("O");
	else System.out.print(".");
	}
	}
	
	private static void fill(boolean[] ob, boolean res){
		for(int i=0; i<ob.length; i++){
			ob[i] = res;
		}
		return;
	}
	private static void fill(int[] ob, int res){
		for(int i=0; i<ob.length; i++){
			ob[i] = res;
		}
		return;
	}
	private static void fill(long[] ob, long res){
		for(int i=0; i<ob.length; i++){
			ob[i] = res;
		}
		return;
	}
	private static void fill(int[][] ob, int res){
		for(int i=0; i<ob.length; i++){
			for(int j=0; j<ob[0].length; j++){
				ob[i][j] = res;
			}
		}
		return;
	}
	private static void fill(long[][] ob, long res){
		for(int i=0; i<ob.length; i++){
			for(int j=0; j<ob[0].length; j++){
				ob[i][j] = res;
			}
		}
		return;
	}
	private static void fill(int[][][] ob, int res){
		for(int i=0; i<ob.length; i++){
			for(int j=0; j<ob[0].length; j++){
				for(int k=0; k<ob[0][0].length; k++){
			ob[i][j][k] = res;
				}
			}
		}
		return;
	}
	private static void fill(long[][][][] ob, long res){
		for(int i=0; i<ob.length; i++){
			for(int j=0; j<ob[0].length; j++){
				for(int k=0; k<ob[0][0].length; k++){
				for(int l=0; l<ob[0][0][0].length; l++){
				ob[i][j][k][l]=res;
				}
				}
			}
		}
		return;
	}
	
	static void show_b2d_array(boolean[][] dt, int lit_x, int lit_y){
		for(int i=0; i<dt[0].length; i++){
			for(int j=0; j<dt.length; j++){
				if((i==lit_y) && (j==lit_x)) System.out.print("X");
				else if(dt[j][i]) System.out.print("O");
				else System.out.print(".");
			}
			System.out.println();
		}
	}
	static void show_L2(long[][] dt, String cmnt){
		PrintWriter out = new PrintWriter(System.out);
		for(int i=0; i<dt[0].length; i++){
			for(int j=0; j<dt.length; j++){
				out.print(dt[j][i]+",");
			}
			out.println("<-"+cmnt+":"+i);
		}
		out.flush();
	}
	static void disp_que(ArrayDeque<Integer> dt){		//上手くいかなかった時用
		int a=0;
		while(dt.size()>0){
			a=dt.removeLast();
			System.out.print(a);
		}
		System.out.println("\n");
	}
	
	
	private static void nextPerm(ArrayList<Integer> former){
		int pivot=-1;
		int ln = former.size();
		for(int i = ln; i>1; i--){
			if(former.get(i-2) < former.get(i-1)){
				pivot = i-2;
				break;
			}
		}
		if(pivot==-1){
		former = null;
		return;
		}
		int pivot_fig = former.get(pivot);
		
		int min_pos=pivot+1;
		int min=former.get(min_pos);
		for(int i=ln-1; i > pivot+1; i--){
			if((former.get(i) < min)&&(former.get(i) > pivot_fig)){
				min_pos=i;
				min=former.get(min_pos);
			}
		}
		Collections.swap(former, pivot, min_pos);
		Collections.sort(former.subList(pivot+1, ln));
		return;
	}
	
	public static int binarySearch(int[] dt, int target){
		int left=0, right=dt.length-1;
		int mid=-1;
		while(left<=right){
			mid = (right+left)/2;
			if(dt[mid] == target) return mid;
			if(dt[mid] < target) left=mid+1;
			else right=mid-1;
		}
		return -1;
	}
	public static int binarySearchMax(long[] dt, long target){
		int left=-1, right=dt.length;
		int mid=-1;
		
		while((right-left)>1){
			mid = left + (right-left)/2;
			if(dt[mid] <= target) left=mid;
			else right=mid;
		}
		return left;//target以下の最大のaddress
	}
	
	
	static int dfs2(boolean[][] state, int x, int y, int depth){		//薄氷渡り
		int[] tmpltX = {1,-1,0,0};
		int[] tmpltY = {0,0,1,-1};
		int newX = -1, newY = -1;
		
		state[x][y] = false;
		depth++;
		
		int maxDunder_this_brunch = depth, tmpD=0;
		for(int i=0; i < 4; i++){
			newX = x+tmpltX[i];
			newY = y+tmpltY[i];
			if((newX >= 0) && (newY >= 0) && (newX < state.length) && (newY < state[0].length) && state[newX][newY]) tmpD = dfs2(state, newX, newY, depth);
			if(tmpD > maxDunder_this_brunch) maxDunder_this_brunch=tmpD;
		}
		state[x][y] = true;
		return maxDunder_this_brunch;
	}
}
 
class FastScanner {
	private final InputStream in = System.in;
	private final byte[] buffer = new byte[1024];
	private int ptr = 0;
	private int buflen = 0;
	private boolean hasNextByte() {
		if (ptr < buflen) {
			return true;
		}else{
			ptr = 0;
			try {
				buflen = in.read(buffer);
			} catch (IOException e) {
				e.printStackTrace();
			}
			if (buflen <= 0) {
				return false;
			}
		}
		return true;
	}
	
	private int readByte() { if (hasNextByte()) return buffer[ptr++]; else return -1;}
	private static boolean isPrintableChar(int c) { return 33 <= c && c <= 126;}
	public boolean hasNext() { while(hasNextByte() && !isPrintableChar(buffer[ptr])) ptr++; return hasNextByte();}
	public String next() {
		if (!hasNext()) throw new NoSuchElementException();
		StringBuilder sb = new StringBuilder();
		int b = readByte();
		while(isPrintableChar(b)) {
			sb.appendCodePoint(b);
			b = readByte();
		}
		return sb.toString();
	}
	public long nextLong() {
		if (!hasNext()) throw new NoSuchElementException();
		long n = 0;
		boolean minus = false;
		int b = readByte();
		if (b == '-') {
			minus = true;
			b = readByte();
		}
		if (b < '0' || '9' < b) {
			throw new NumberFormatException();
		}
		while(true){
			if ('0' <= b && b <= '9') {
				n *= 10;
				n += b - '0';
			}else if(b == -1 || !isPrintableChar(b)){
				return minus ? -n : n;
			}else{
				throw new NumberFormatException();
			}
			b = readByte();
		}
	}
	public int nextInt() {
		long nl = nextLong();
		if (nl < Integer.MIN_VALUE || nl > Integer.MAX_VALUE) throw new NumberFormatException();
		return (int) nl;
	}
	public double nextDouble() { return Double.parseDouble(next());}
}