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