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[] A =new int[M];;
   int sum=0;
   
   for(int i=0;i<M;i++){
      A[i] = sc.nextInt();
      sum=sum+A[i];
   }

   if(sum<=N){
     System.out.println(N-sum);
   }
   else{
     System.out.println(-1);
   }
 
 }
} import java.util.Scanner; Scanner java.util class Main{
 public static void main(String args[]){
   Scanner sc = new Scanner(System.in);
   int N = sc.nextInt();
   int M = sc.nextInt();
   int[] A =new int[M];;
   int sum=0;
   
   for(int i=0;i<M;i++){
      A[i] = sc.nextInt();
      sum=sum+A[i];
   }

   if(sum<=N){
     System.out.println(N-sum);
   }
   else{
     System.out.println(-1);
   }
 
 }
} Main public static void main(String args[]){
   Scanner sc = new Scanner(System.in);
   int N = sc.nextInt();
   int M = sc.nextInt();
   int[] A =new int[M];;
   int sum=0;
   
   for(int i=0;i<M;i++){
      A[i] = sc.nextInt();
      sum=sum+A[i];
   }

   if(sum<=N){
     System.out.println(N-sum);
   }
   else{
     System.out.println(-1);
   }
 
 } main {
   Scanner sc = new Scanner(System.in);
   int N = sc.nextInt();
   int M = sc.nextInt();
   int[] A =new int[M];;
   int sum=0;
   
   for(int i=0;i<M;i++){
      A[i] = sc.nextInt();
      sum=sum+A[i];
   }

   if(sum<=N){
     System.out.println(N-sum);
   }
   else{
     System.out.println(-1);
   }
 
 } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int N = sc.nextInt(); N sc.nextInt() sc.nextInt sc int M = sc.nextInt(); M sc.nextInt() sc.nextInt sc int[] A =new int[M]; A new int[M] M int sum=0; sum 0 for(int i=0;i<M;i++){
      A[i] = sc.nextInt();
      sum=sum+A[i];
   } int i=0; int i=0; i 0 i<M i M i++ i++ i {
      A[i] = sc.nextInt();
      sum=sum+A[i];
   } {
      A[i] = sc.nextInt();
      sum=sum+A[i];
   } A[i] = sc.nextInt() A[i] A i sc.nextInt() sc.nextInt sc sum=sum+A[i] sum sum+A[i] sum A[i] A i if(sum<=N){
     System.out.println(N-sum);
   }
   else{
     System.out.println(-1);
   } sum<=N sum N {
     System.out.println(N-sum);
   } System.out.println(N-sum) System.out.println System.out System System.out N-sum N sum {
     System.out.println(-1);
   } System.out.println(-1) System.out.println System.out System System.out -1 1 String args[] args
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[] A =new int[M];;
   int sum=0;
   
   for(int i=0;i<M;i++){
      A[i] = sc.nextInt();
      sum=sum+A[i];
   }

   if(sum<=N){
     System.out.println(N-sum);
   }
   else{
     System.out.println(-1);
   }
 
 }
}
