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