import java.util.Scanner;

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();

		System.out.println(sum > n ? -1 : n - sum);
	}
} import java.util.Scanner; Scanner 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();

		System.out.println(sum > n ? -1 : n - sum);
	}
} 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();

		System.out.println(sum > n ? -1 : n - sum);
	} 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();

		System.out.println(sum > n ? -1 : n - sum);
	} 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() sc.nextInt sc 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) {

		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();

		System.out.println(sum > n ? -1 : n - sum);
	}
} 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();

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

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();

		System.out.println(sum > n ? -1 : n - sum);
	}
}