import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        int M = sc.nextInt();
        int N = sc.nextInt();
        int[] A = new int[N];

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

        System.out.println(M >= 0 ? M : -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