/* package codechef; // don't place package name! */

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



/* Name of the class has to be "Main" only if the class is public. */
public class Main
 
 
{
    static Scanner in = new Scanner(System.in);
    
    public static void solve()
    {
        int n=in.nextInt(),m=in.nextInt();
        int[] a = new int[m];
        for(int i=0;i<m;i++)
        {
            a[i]=in.nextInt();
        }
        
        int sum=0;
        
        for(int i=0;i<m;i++)
        {
            sum+=a[i];
            
            if(sum>n)
            {
                break;
            }
            
        }
        
        if(sum>n)
        {
            System.out.println(-1);
        }
        else
        {
            System.out.println(n-sum);
        }
        
    }
    

    
	public static void main (String[] args) throws java.lang.Exception
	{
		// your code goes here
		
		int t=1;
		//in.nextLine();
		
		while(t>0)
		{
		    
		    solve();
		    t--;
		}
		
		
	
	//	System.out.println(x);
	}
} import java.util.*; util.* java import java.lang.*; lang.* java import java.io.*; io.* java public class Main
 
 
{
    static Scanner in = new Scanner(System.in);
    
    public static void solve()
    {
        int n=in.nextInt(),m=in.nextInt();
        int[] a = new int[m];
        for(int i=0;i<m;i++)
        {
            a[i]=in.nextInt();
        }
        
        int sum=0;
        
        for(int i=0;i<m;i++)
        {
            sum+=a[i];
            
            if(sum>n)
            {
                break;
            }
            
        }
        
        if(sum>n)
        {
            System.out.println(-1);
        }
        else
        {
            System.out.println(n-sum);
        }
        
    }
    

    
	public static void main (String[] args) throws java.lang.Exception
	{
		// your code goes here
		
		int t=1;
		//in.nextLine();
		
		while(t>0)
		{
		    
		    solve();
		    t--;
		}
		
		
	
	//	System.out.println(x);
	}
} Main static Scanner in = new Scanner(System.in); in new Scanner(System.in) public static void solve()
    {
        int n=in.nextInt(),m=in.nextInt();
        int[] a = new int[m];
        for(int i=0;i<m;i++)
        {
            a[i]=in.nextInt();
        }
        
        int sum=0;
        
        for(int i=0;i<m;i++)
        {
            sum+=a[i];
            
            if(sum>n)
            {
                break;
            }
            
        }
        
        if(sum>n)
        {
            System.out.println(-1);
        }
        else
        {
            System.out.println(n-sum);
        }
        
    } solve {
        int n=in.nextInt(),m=in.nextInt();
        int[] a = new int[m];
        for(int i=0;i<m;i++)
        {
            a[i]=in.nextInt();
        }
        
        int sum=0;
        
        for(int i=0;i<m;i++)
        {
            sum+=a[i];
            
            if(sum>n)
            {
                break;
            }
            
        }
        
        if(sum>n)
        {
            System.out.println(-1);
        }
        else
        {
            System.out.println(n-sum);
        }
        
    } int n=in.nextInt() n in.nextInt() in.nextInt in m=in.nextInt(); m in.nextInt() in.nextInt in int[] a = new int[m]; a new int[m] m for(int i=0;i<m;i++)
        {
            a[i]=in.nextInt();
        } int i=0; int i=0; i 0 i<m i m i++ i++ i {
            a[i]=in.nextInt();
        } {
            a[i]=in.nextInt();
        } a[i]=in.nextInt() a[i] a i in.nextInt() in.nextInt in int sum=0; sum 0 for(int i=0;i<m;i++)
        {
            sum+=a[i];
            
            if(sum>n)
            {
                break;
            }
            
        } int i=0; int i=0; i 0 i<m i m i++ i++ i {
            sum+=a[i];
            
            if(sum>n)
            {
                break;
            }
            
        } {
            sum+=a[i];
            
            if(sum>n)
            {
                break;
            }
            
        } sum+=a[i] sum a[i] a i if(sum>n)
            {
                break;
            } sum>n sum n {
                break;
            } break; if(sum>n)
        {
            System.out.println(-1);
        }
        else
        {
            System.out.println(n-sum);
        } sum>n sum n {
            System.out.println(-1);
        } System.out.println(-1) System.out.println System.out System System.out -1 1 {
            System.out.println(n-sum);
        } System.out.println(n-sum) System.out.println System.out System System.out n-sum n sum public static void main (String[] args) throws java.lang.Exception
	{
		// your code goes here
		
		int t=1;
		//in.nextLine();
		
		while(t>0)
		{
		    
		    solve();
		    t--;
		}
		
		
	
	//	System.out.println(x);
	} main {
		// your code goes here
		
		int t=1;
		//in.nextLine();
		
		while(t>0)
		{
		    
		    solve();
		    t--;
		}
		
		
	
	//	System.out.println(x);
	} int t=1; t 1 while(t>0)
		{
		    
		    solve();
		    t--;
		} t>0 t 0 {
		    
		    solve();
		    t--;
		} solve() solve t-- t String[] args args public class Main
 
 
{
    static Scanner in = new Scanner(System.in);
    
    public static void solve()
    {
        int n=in.nextInt(),m=in.nextInt();
        int[] a = new int[m];
        for(int i=0;i<m;i++)
        {
            a[i]=in.nextInt();
        }
        
        int sum=0;
        
        for(int i=0;i<m;i++)
        {
            sum+=a[i];
            
            if(sum>n)
            {
                break;
            }
            
        }
        
        if(sum>n)
        {
            System.out.println(-1);
        }
        else
        {
            System.out.println(n-sum);
        }
        
    }
    

    
	public static void main (String[] args) throws java.lang.Exception
	{
		// your code goes here
		
		int t=1;
		//in.nextLine();
		
		while(t>0)
		{
		    
		    solve();
		    t--;
		}
		
		
	
	//	System.out.println(x);
	}
} public class Main
 
 
{
    static Scanner in = new Scanner(System.in);
    
    public static void solve()
    {
        int n=in.nextInt(),m=in.nextInt();
        int[] a = new int[m];
        for(int i=0;i<m;i++)
        {
            a[i]=in.nextInt();
        }
        
        int sum=0;
        
        for(int i=0;i<m;i++)
        {
            sum+=a[i];
            
            if(sum>n)
            {
                break;
            }
            
        }
        
        if(sum>n)
        {
            System.out.println(-1);
        }
        else
        {
            System.out.println(n-sum);
        }
        
    }
    

    
	public static void main (String[] args) throws java.lang.Exception
	{
		// your code goes here
		
		int t=1;
		//in.nextLine();
		
		while(t>0)
		{
		    
		    solve();
		    t--;
		}
		
		
	
	//	System.out.println(x);
	}
} Main
/* package codechef; // don't place package name! */

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



/* Name of the class has to be "Main" only if the class is public. */
public class Main
 
 
{
    static Scanner in = new Scanner(System.in);
    
    public static void solve()
    {
        int n=in.nextInt(),m=in.nextInt();
        int[] a = new int[m];
        for(int i=0;i<m;i++)
        {
            a[i]=in.nextInt();
        }
        
        int sum=0;
        
        for(int i=0;i<m;i++)
        {
            sum+=a[i];
            
            if(sum>n)
            {
                break;
            }
            
        }
        
        if(sum>n)
        {
            System.out.println(-1);
        }
        else
        {
            System.out.println(n-sum);
        }
        
    }
    

    
	public static void main (String[] args) throws java.lang.Exception
	{
		// your code goes here
		
		int t=1;
		//in.nextLine();
		
		while(t>0)
		{
		    
		    solve();
		    t--;
		}
		
		
	
	//	System.out.println(x);
	}
}
