import java.util.*;

public class Main{
    public static void main(String[] args){
	Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    int M = sc.nextInt();
	int sum = 0;
    for(int i = 0 ; i < M ; i++){
      sum += sc.nextInt();
    }
      if(N < sum)
        System.out.println(-1);
      else 
        System.out.println(N - sum);
        
	
	}
}
/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