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.IOException;
import java.io.InputStreamReader;
import java.io.BufferedReader;
import java.util.*;
import java.math.*;
import java.io.*;
import java.text.*;
import java.math.BigInteger;
 
public class Main
{  
 
    //Life's a bitch
    public static boolean[] sieve(long n)
    {
        boolean[] prime = new boolean[(int)n+1];
        Arrays.fill(prime,true);
        prime[0] = false;
        prime[1] = false;
        long m = (long)Math.sqrt(n);
        for(int i=2;i<=m;i++)
        {
            if(prime[i])
            {
                for(int k=i*i;k<=n;k+=i)
                {
                    prime[k] = false;
                }
            }
        }
        return prime;
    } 
 
    
    static long GCD(long a,long b)
    {
        if(b==0)
        {
            return a;
        }
        return GCD(b,a%b);
    }
 
    static long CountCoPrimes(long n)
    {
        long res = n;
        for(int i=2;i*i<=n;i++)
        {
            if(n%i==0)
            {
                while(n%i==0)
                {
                    n/=i;
                }
                res-=res/i;
            }
        }
        if(n>1)
        {
            res-=res/n;
        }
        return res;
    }
 
 
    
 
    
 
    static boolean prime(int n)
    {
        for(int i=2;i*i<=n;i++)
        {
            if(n%i==0)
            {
                return false;
            }
        }
        return true;
    }

    
 
   
 
 
   
   
  
    public static void main(String[] args) throws IOException
    {
     
        new Main().run();
       
    }
 
 
    
    static long LCM(long a,long b)
    {
        return (a*b)/GCD(a,b);
    }
   
 
    Scanner in = new Scanner(System.in);
    // static int[] arr,tree[],lev;
    void run() throws IOException
    {
       long n = nl();
       long m = nl();
       long tot = 0;
       for(int i=0;i<m;i++)
       {
            tot+=nl();
       } 

       if(tot>n)
       {
            printL(-1);
            return;
       }
       printL(n-tot);
            
         
       
      


           

    
    }





   
    




   
    
    


    




  

    

    

    
    


    
    


   

 

    
 
 
 
 
 
 
    
 
    //xor range query
    static long xor(long n)
    {
 
        if(n%4==0)
        {
            return n;
        }
        if(n%4==1)
        {
            return  1;
        }
        if(n%4==2)
        {
            return n+1;
        }
        return 0;
    }
 
    static long xor(long a,long b)
    {
        return xor(b)^xor(a-1);
    }
 
 
    
 
  
   
    
 
    
       
    
 
    
   
  
 
 
 
   
 
     void printL(long a)
    {
        System.out.println(a);
    }
    void printS(String s)
    {
        System.out.println(s);
    }
 
     void printD(Double d)
    {
        System.out.println(d);
    }
  
   
   
    static void swap(char[] c,int a,int b)
    {
        char t = c[a];
        c[a] = c[b];
        c[b] = t;
    }
 
    static long max(long n,long m)
    {
        return Math.max(n,m);
    }
    static long min(long n,long m)
    {
        return Math.min(n,m);
    }
 
    double nd() throws IOException
    {
        return Double.parseDouble(in.next());
    }
    int ni() throws IOException
    {
        return Integer.parseInt(in.next());
    }
 
    long nl() throws IOException
    {
        return Long.parseLong(in.next());
    }
 
    String si() throws IOException
    {
        return in.next();
    }
 
 
  
   
    static long abs(long n)
    {
        return Math.abs(n);
    }
 
    static class Scanner 
    {
        StringTokenizer st;
        BufferedReader br;
 
        public Scanner(InputStream s){  br = new BufferedReader(new InputStreamReader(s));}
 
        public String next() throws IOException 
        {
            while (st == null || !st.hasMoreTokens()) 
            {
                st = new StringTokenizer(br.readLine());
            }
            return st.nextToken();
        }
 
        public int nextInt() throws IOException {return Integer.parseInt(next());}
        
        public long nextLong() throws IOException {return Long.parseLong(next());}
 
        public String nextLine() throws IOException {return br.readLine();}
 
        public boolean ready() throws IOException {return br.ready();}
 
 
    }
 
    
}
 
 
 
class Pair implements Comparable<Pair>
{
   int x,y;
    public Pair(int x,int y)
    {
        this.x = x;
        this.y = y;
    }
    public int compareTo(Pair o)
    {
        return this.y-o.y;
    }
}