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);
        }
    }
}
/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz