import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        @SuppressWarnings("resource")
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int m = sc.nextInt();
        int [] a = new int[m];
        for (int i = 0 ; i < m ; i++) {
            a[i] = sc.nextInt();
        }
        int sum = 0;
        for (int i = 0 ; i < m ; i++) {
            sum += a[i];
        }
        System.out.println(Math.max(-1, n - sum));
    }

} import java.util.Scanner; Scanner java.util public class Main {

    public static void main(String[] args) {
        @SuppressWarnings("resource")
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int m = sc.nextInt();
        int [] a = new int[m];
        for (int i = 0 ; i < m ; i++) {
            a[i] = sc.nextInt();
        }
        int sum = 0;
        for (int i = 0 ; i < m ; i++) {
            sum += a[i];
        }
        System.out.println(Math.max(-1, n - sum));
    }

} Main public static void main(String[] args) {
        @SuppressWarnings("resource")
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int m = sc.nextInt();
        int [] a = new int[m];
        for (int i = 0 ; i < m ; i++) {
            a[i] = sc.nextInt();
        }
        int sum = 0;
        for (int i = 0 ; i < m ; i++) {
            sum += a[i];
        }
        System.out.println(Math.max(-1, n - sum));
    } main {
        @SuppressWarnings("resource")
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int m = sc.nextInt();
        int [] a = new int[m];
        for (int i = 0 ; i < m ; i++) {
            a[i] = sc.nextInt();
        }
        int sum = 0;
        for (int i = 0 ; i < m ; i++) {
            sum += a[i];
        }
        System.out.println(Math.max(-1, n - sum));
    } @SuppressWarnings("resource")
        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 [] 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 = 0; sum 0 for (int i = 0 ; i < m ; i++) {
            sum += a[i];
        } int i = 0 ; int i = 0 ; i 0 i < m i m i++ i++ i {
            sum += a[i];
        } {
            sum += a[i];
        } sum += a[i] sum a[i] a i System.out.println(Math.max(-1, n - sum)) System.out.println System.out System System.out Math.max(-1, n - sum) Math.max Math -1 1 n - sum n sum String[] args args public class Main {

    public static void main(String[] args) {
        @SuppressWarnings("resource")
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int m = sc.nextInt();
        int [] a = new int[m];
        for (int i = 0 ; i < m ; i++) {
            a[i] = sc.nextInt();
        }
        int sum = 0;
        for (int i = 0 ; i < m ; i++) {
            sum += a[i];
        }
        System.out.println(Math.max(-1, n - sum));
    }

} public class Main {

    public static void main(String[] args) {
        @SuppressWarnings("resource")
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int m = sc.nextInt();
        int [] a = new int[m];
        for (int i = 0 ; i < m ; i++) {
            a[i] = sc.nextInt();
        }
        int sum = 0;
        for (int i = 0 ; i < m ; i++) {
            sum += a[i];
        }
        System.out.println(Math.max(-1, n - sum));
    }

} Main
import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        @SuppressWarnings("resource")
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        int m = sc.nextInt();
        int [] a = new int[m];
        for (int i = 0 ; i < m ; i++) {
            a[i] = sc.nextInt();
        }
        int sum = 0;
        for (int i = 0 ; i < m ; i++) {
            sum += a[i];
        }
        System.out.println(Math.max(-1, n - sum));
    }

}
