import java.util.*;
public class Main {
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		// 整数の入力(スペース改行関係なし)
		int n = sc.nextInt();
		int m = sc.nextInt();
        int syukudai = 0;
      	int asobi = 0;
      
      for(int i = 0; i < m ; i++){
      	syukudai += sc.nextInt();
      
      }
      asobi = n - syukudai;
		if(asobi < 0){
			// 出力
			System.out.println(-1);
		}else{
			System.out.println(asobi);
		}
		
	}
}
/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