import java.util.Arrays;
import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        final Scanner sc = new Scanner(System.in);

        final int n = sc.nextInt();
        final int m = sc.nextInt();

        final int[] a = new int[m];
        for (int i = 0; i < m; i++) {
            a[i] = sc.nextInt();
        }

        int sum = Arrays.stream(a).sum();
        System.out.println(sum > n ? -1 : n - sum);
    }
} import java.util.Arrays; Arrays java.util import java.util.Scanner; Scanner java.util public class Main {

    public static void main(String[] args) {
        final Scanner sc = new Scanner(System.in);

        final int n = sc.nextInt();
        final int m = sc.nextInt();

        final int[] a = new int[m];
        for (int i = 0; i < m; i++) {
            a[i] = sc.nextInt();
        }

        int sum = Arrays.stream(a).sum();
        System.out.println(sum > n ? -1 : n - sum);
    }
} Main public static void main(String[] args) {
        final Scanner sc = new Scanner(System.in);

        final int n = sc.nextInt();
        final int m = sc.nextInt();

        final int[] a = new int[m];
        for (int i = 0; i < m; i++) {
            a[i] = sc.nextInt();
        }

        int sum = Arrays.stream(a).sum();
        System.out.println(sum > n ? -1 : n - sum);
    } main {
        final Scanner sc = new Scanner(System.in);

        final int n = sc.nextInt();
        final int m = sc.nextInt();

        final int[] a = new int[m];
        for (int i = 0; i < m; i++) {
            a[i] = sc.nextInt();
        }

        int sum = Arrays.stream(a).sum();
        System.out.println(sum > n ? -1 : n - sum);
    } final Scanner sc = new Scanner(System.in); sc new Scanner(System.in) final int n = sc.nextInt(); n sc.nextInt() sc.nextInt sc final int m = sc.nextInt(); m sc.nextInt() sc.nextInt sc final int[] a = new int[m]; a new int[m] m for (int i = 0; i < m; i++) {
            a[i] = sc.nextInt();
        } int i = 0; int i = 0; i 0 i < m i m i++ i++ i {
            a[i] = sc.nextInt();
        } {
            a[i] = sc.nextInt();
        } a[i] = sc.nextInt() a[i] a i sc.nextInt() sc.nextInt sc int sum = Arrays.stream(a).sum(); sum Arrays.stream(a).sum() Arrays.stream(a).sum Arrays.stream(a) Arrays.stream Arrays a System.out.println(sum > n ? -1 : n - sum) System.out.println System.out System System.out sum > n ? -1 : n - sum sum > n sum n -1 1 n - sum n sum String[] args args public class Main {

    public static void main(String[] args) {
        final Scanner sc = new Scanner(System.in);

        final int n = sc.nextInt();
        final int m = sc.nextInt();

        final int[] a = new int[m];
        for (int i = 0; i < m; i++) {
            a[i] = sc.nextInt();
        }

        int sum = Arrays.stream(a).sum();
        System.out.println(sum > n ? -1 : n - sum);
    }
} public class Main {

    public static void main(String[] args) {
        final Scanner sc = new Scanner(System.in);

        final int n = sc.nextInt();
        final int m = sc.nextInt();

        final int[] a = new int[m];
        for (int i = 0; i < m; i++) {
            a[i] = sc.nextInt();
        }

        int sum = Arrays.stream(a).sum();
        System.out.println(sum > n ? -1 : n - sum);
    }
} Main
import java.util.Arrays;
import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        final Scanner sc = new Scanner(System.in);

        final int n = sc.nextInt();
        final int m = sc.nextInt();

        final int[] a = new int[m];
        for (int i = 0; i < m; i++) {
            a[i] = sc.nextInt();
        }

        int sum = Arrays.stream(a).sum();
        System.out.println(sum > n ? -1 : n - sum);
    }
}
