import 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[] a = new int[m];

        for(int i = 0; i < m; i++)
            a[i] = sc.nextInt();

        int ans = n;
        for(int i = 0; i < m; i++)
            ans = ans - a[i];

        if(ans < 0){
            System.out.println(-1);
        }else{
            System.out.println(ans);
        }
    }
}