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[] A = new int[M];
	int sNum = 0;
	for(int i=0;i<M;i++){
	    A[i] = sc.nextInt();
	    sNum += A[i];
	}
	if(N - sNum >= 0) System.out.print(N - sNum);
	else System.out.print(-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