import java.util.Scanner;

class Main {
	public static void main(String[] $){
		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();
		}

		if (n < sum) {
			System.out.println("-1");
		} else {
			System.out.println(n - sum);
		}

		sc.close();
	}
}
 import java.util.Scanner; Scanner java.util class Main {
	public static void main(String[] $){
		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();
		}

		if (n < sum) {
			System.out.println("-1");
		} else {
			System.out.println(n - sum);
		}

		sc.close();
	}
} Main public static void main(String[] $){
		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();
		}

		if (n < sum) {
			System.out.println("-1");
		} else {
			System.out.println(n - sum);
		}

		sc.close();
	} 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();
		}

		if (n < sum) {
			System.out.println("-1");
		} else {
			System.out.println(n - sum);
		}

		sc.close();
	} 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 if (n < sum) {
			System.out.println("-1");
		} else {
			System.out.println(n - sum);
		} n < sum n sum {
			System.out.println("-1");
		} System.out.println("-1") System.out.println System.out System System.out "-1" {
			System.out.println(n - sum);
		} System.out.println(n - sum) System.out.println System.out System System.out n - sum n sum sc.close() sc.close sc String[] $ $
import java.util.Scanner;

class Main {
	public static void main(String[] $){
		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();
		}

		if (n < sum) {
			System.out.println("-1");
		} else {
			System.out.println(n - sum);
		}

		sc.close();
	}
}

