import java.util.*;
public class Main {
    public static void main(String[] args){
        Scanner scan = new Scanner(System.in);
        int n = scan.nextInt();
        int m = scan.nextInt();
        int a[] = new int[m];
        for(int i = 0; i < m; i++){
            a[i] = scan.nextInt();
        }
        
        for(int i = 0; i < m; i++){
            n = n - a[i];
        }

        if(n < 0){
            System.out.println("-1");
        }else{
            System.out.println(n);
        }
    }
}
/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