import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;


public class Main {

    public static void main(String[] args) throws IOException {
        BufferedReader reader=new BufferedReader(new InputStreamReader(System.in));
        String[] s=reader.readLine().split(" ");
        int n=Integer.parseInt(s[0]);
        int m=Integer.parseInt(s[1]);
        String[] s1=reader.readLine().split(" ");
        int[] days=new int[m];
        int count=0;
        for (int i = 0; i <m ; i++) {
            days[i]=Integer.parseInt(s1[i]);
            count+=days[i];
        }
        if (count<=n)
            System.out.println(n-count);
        else if (count>n)
            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