import java.util.Scanner;

public class Main
{
    public  static  void main(String[] args)
    {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        int M = sc.nextInt();
        int sum = 0;
        int arr[] = new int[M];
        for(int i=0;i<M;i++)
        {
            arr[i] = sc.nextInt();
            sum = sum+arr[i];
        }

        if(N-sum>=0)
        {System.out.print((N-sum));}
        else
        {System.out.print(-1);}


    }
} import java.util.Scanner; Scanner java.util public class Main
{
    public  static  void main(String[] args)
    {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        int M = sc.nextInt();
        int sum = 0;
        int arr[] = new int[M];
        for(int i=0;i<M;i++)
        {
            arr[i] = sc.nextInt();
            sum = sum+arr[i];
        }

        if(N-sum>=0)
        {System.out.print((N-sum));}
        else
        {System.out.print(-1);}


    }
} Main public  static  void main(String[] args)
    {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        int M = sc.nextInt();
        int sum = 0;
        int arr[] = new int[M];
        for(int i=0;i<M;i++)
        {
            arr[i] = sc.nextInt();
            sum = sum+arr[i];
        }

        if(N-sum>=0)
        {System.out.print((N-sum));}
        else
        {System.out.print(-1);}


    } main {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        int M = sc.nextInt();
        int sum = 0;
        int arr[] = new int[M];
        for(int i=0;i<M;i++)
        {
            arr[i] = sc.nextInt();
            sum = sum+arr[i];
        }

        if(N-sum>=0)
        {System.out.print((N-sum));}
        else
        {System.out.print(-1);}


    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int N = sc.nextInt(); N sc.nextInt() sc.nextInt sc int M = sc.nextInt(); M sc.nextInt() sc.nextInt sc int sum = 0; sum 0 int arr[] = new int[M]; arr new int[M] M for(int i=0;i<M;i++)
        {
            arr[i] = sc.nextInt();
            sum = sum+arr[i];
        } int i=0; int i=0; i 0 i<M i M i++ i++ i {
            arr[i] = sc.nextInt();
            sum = sum+arr[i];
        } {
            arr[i] = sc.nextInt();
            sum = sum+arr[i];
        } arr[i] = sc.nextInt() arr[i] arr i sc.nextInt() sc.nextInt sc sum = sum+arr[i] sum sum+arr[i] sum arr[i] arr i if(N-sum>=0)
        {System.out.print((N-sum));}
        else
        {System.out.print(-1);} N-sum>=0 N-sum N sum 0 {System.out.print((N-sum));} System.out.print((N-sum)) System.out.print System.out System System.out (N-sum) N sum {System.out.print(-1);} System.out.print(-1) System.out.print System.out System System.out -1 1 String[] args args public class Main
{
    public  static  void main(String[] args)
    {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        int M = sc.nextInt();
        int sum = 0;
        int arr[] = new int[M];
        for(int i=0;i<M;i++)
        {
            arr[i] = sc.nextInt();
            sum = sum+arr[i];
        }

        if(N-sum>=0)
        {System.out.print((N-sum));}
        else
        {System.out.print(-1);}


    }
} public class Main
{
    public  static  void main(String[] args)
    {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        int M = sc.nextInt();
        int sum = 0;
        int arr[] = new int[M];
        for(int i=0;i<M;i++)
        {
            arr[i] = sc.nextInt();
            sum = sum+arr[i];
        }

        if(N-sum>=0)
        {System.out.print((N-sum));}
        else
        {System.out.print(-1);}


    }
} Main
import java.util.Scanner;

public class Main
{
    public  static  void main(String[] args)
    {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        int M = sc.nextInt();
        int sum = 0;
        int arr[] = new int[M];
        for(int i=0;i<M;i++)
        {
            arr[i] = sc.nextInt();
            sum = sum+arr[i];
        }

        if(N-sum>=0)
        {System.out.print((N-sum));}
        else
        {System.out.print(-1);}


    }
}
