import java.util.*;

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

public class 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=0; i<m; i++){
            n -= sc.nextInt();
        }
        
        System.out.println(n>=0?n:-1);
    }
}