import java.util.*;
public class Main {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner in = new Scanner(System.in);
	    int n = in.nextInt();
	    int m = in.nextInt();
        int ar[] = new int[n];
      	int sum=0;
      	for(int i=0;i<m;i++){
          ar[i]=in.nextInt();
          sum+=ar[i];
        }
      	int res = n-sum;
      	
        if(res>=0)
          System.out.println(res);
        else
          System.out.println("-1");
          
	}

}

/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