
import java.util.*;

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

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

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

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

	
} Main

import java.util.*;

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

	
}