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