import java.util.Scanner;
import java.lang.Math;
class Main{
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt(),
            m = sc.nextInt();
        int A = 0;
        for(int i=0;i<m;i++){
            A = A + sc.nextInt();
        }
        if(A <= n){
            System.out.print(n-A);
        }else{
            System.out.print(-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