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