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();
        }

        int res = n - sum;
        if(res < 0) res = -1;

        System.out.println(res);


    }
} 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();
        }

        int res = n - sum;
        if(res < 0) res = -1;

        System.out.println(res);


    }
} 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();
        }

        int res = n - sum;
        if(res < 0) res = -1;

        System.out.println(res);


    } 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();
        }

        int res = n - sum;
        if(res < 0) res = -1;

        System.out.println(res);


    } 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 int res = n - sum; res n - sum n sum if(res < 0) res = -1; res < 0 res 0 res = -1 res -1 1 System.out.println(res) System.out.println System.out System System.out res 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();
        }

        int res = n - sum;
        if(res < 0) res = -1;

        System.out.println(res);


    }
} 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();
        }

        int res = n - sum;
        if(res < 0) res = -1;

        System.out.println(res);


    }
} 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();
        }

        int res = n - sum;
        if(res < 0) res = -1;

        System.out.println(res);


    }
}