import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.*;
 
public class Main {
    public static void main(String args[]) throws IOException {
	Scanner scanner = new Scanner(System.in);
	int summer = scanner.nextInt();
	//宿題の数
	int num = scanner.nextInt();
	//計算用
      int tmp = 0;
      
	for(int i=0;i<num;i++){
        tmp += scanner.nextInt();
	}
	
    int day = summer - tmp;
      
    if(day>=0){
      System.out.println(day);
    }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