import java.util.Scanner;

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++){
  	    	int A = sc.nextInt();
  	    	sum += A;
    	}
  		int freeDays = N - sum;
  		if(freeDays < 0){
  	    	freeDays = -1;
    	}
  		System.out.println(freeDays);
    }
}
/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