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



class Main
{

   
    public static void process()throws IOException
    {

                 int n = ni();
                 int m = ni();
                 int sum=0;
                 int a[] = new int[m];
                 for(int i=0;i<m;i++)
                 {
                      a[i] = ni();
                 }
                 for(int i=0;i<m;i++)
                 {
                        sum+=a[i];
                 }
                 if(sum>n)
                 {
                       pn("-1");
                 }
                 else
                 {
                       pn(n-sum);
                 }
     


       

   


    }     

            
           
    




    static AnotherReader sc;
    static PrintWriter out;
    public static void main(String[]args)throws IOException
    {
        out = new PrintWriter(System.out);
        sc=new AnotherReader();
        boolean oj = true;

     //   oj = System.getProperty("ONLINE_JUDGE") != null;
     //   if(!oj) sc=new AnotherReader(100);

       long s = System.currentTimeMillis();
         int t=1;//test cases
         while(t-->0)
            process();
        out.flush();
     //   if(!oj)
     //     System.out.println(System.currentTimeMillis()-s+"ms");
        System.out.close();  
    }

    static void pn(Object o){out.println(o);}
    static void p(Object o){out.print(o);}
    static void pni(Object o){out.println(o);System.out.flush();}
    static int ni()throws IOException{return sc.nextInt();}
    static long nl()throws IOException{return sc.nextLong();}
    static double nd()throws IOException{return sc.nextDouble();}
    static String nln()throws IOException{return sc.nextLine();}
    static long gcd(long a, long b)throws IOException{return (b==0)?a:gcd(b,a%b);}
    static int gcd(int a, int b)throws IOException{return (b==0)?a:gcd(b,a%b);}
    static int bit(long n)throws IOException{return (n==0)?0:(1+bit(n&(n-1)));}
    static boolean multipleTC=false;



/////////////////////////////////////////////////////////////////////////////////////////////////////////

    static class AnotherReader
    {
        BufferedReader br; StringTokenizer st;
        AnotherReader()throws FileNotFoundException
        {
              br=new BufferedReader(new InputStreamReader(System.in));
        }
        AnotherReader(int a)throws FileNotFoundException
        {
              br = new BufferedReader(new FileReader("input.txt"));
        }
        String next()throws IOException
        {
              while (st == null || !st.hasMoreElements()) 
                {
                    try
                     {
                               st = new StringTokenizer(br.readLine());
                     }
                    catch (IOException  e)
                    {
                               e.printStackTrace(); 
                     }
                }
                return st.nextToken(); 
        }
        int nextInt() throws IOException
        {
                return Integer.parseInt(next());
        }
        long nextLong() throws IOException
        {  
                return Long.parseLong(next());
        }
        double nextDouble()throws IOException 
        { 
               return Double.parseDouble(next()); 
        }
        String nextLine() throws IOException
        { 
               String str = ""; 
               try
               {
                     str = br.readLine();
               }
               catch (IOException e)
               {
                          e.printStackTrace();
               } return str;
        }
    }
    
/////////////////////////////////////////////////////////////////////////////////////////////////////////////
}
    
    
    
    

Measured memory_bytes: 34232