import java.util.Scanner;
class Main{
public static void main(String[]args){

Scanner sc=new Scanner(System.in);
int N=sc.nextInt();
int M=sc.nextInt();

int num=0;

for(int n=0;n<M;n++){
int An=sc.nextInt();
N=N-An;
}
if(N>=0){
System.out.println(N);
}else if(N<0){
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