import java.util.Scanner;
class Main {
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner input = new Scanner(System.in); 
      int a = input.nextInt();
      int b =input.nextInt();
      for(int i =0; i<b; i++){
        	int c = input.nextInt();
      		a = a-c;
      }
      if(a<0){
        System.out.println ("-1");
      }
      else{
        System.out.println(a);
      }
    }
}
/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