
import java.util.*;

public class Main {// Main
	public static void main(String[] args) throws Exception {
		Scanner sc = new Scanner(System.in);
   int n = sc.nextInt();
   int m = sc.nextInt();
   for(int i =1 ; i <=m ;i++) {
	   n-=sc.nextInt();
   }
    if(n>=0)
    System.out.println(n);
    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