import java.util.*;
import java.math.*;
import java.io.*;

public class Main{
	public static void main(String[] args) {
		Scanner in = new Scanner(System.in);
		int n = in.nextInt();
		int tt = in.nextInt();
		int sum = 0;
		for(int qq=1; qq<=tt; qq++){
			sum += in.nextInt();
		}
		if(sum<=n){
			System.out.println(n-sum);
		}else{
			System.out.println("-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