import java.util.Scanner;
class Main{
    public static void main(String args[]){
        Scanner scan = new Scanner(System.in);
        int day = scan.nextInt();
        int homework = scan.nextInt();
        int sum = 0;
        int suji[] = new int[homework];
        for(int j = 0; j < suji.length; j++){
            suji[j] = scan.nextInt();
        }
        for(int i = 0; i < suji.length; i++){
            sum += suji[i];
        }
        int canPlay;
        canPlay = day - sum; 
        if(canPlay >=0){
            System.out.println(canPlay);
        }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