import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        //コード
    	Scanner sc = new Scanner(System.in);
    	int N = sc.nextInt();
    	int M = sc.nextInt();

    	int s = 0;

    	for(int i=0;i<M;i++) {
    		s = s + sc.nextInt();
    	}

    	if(N>=s) {
    		System.out.println(N-s);
    	} 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