import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		// TODO 自動生成されたメソッド・スタブ
		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 val:a) {
			sum += val;
		}
		if (n-sum < 0) {
			System.out.println("-1");
		} else {
			System.out.println(n-sum);
		}
		sc.close();

	}

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

	public static void main(String[] args) {
		// TODO 自動生成されたメソッド・スタブ
		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 val:a) {
			sum += val;
		}
		if (n-sum < 0) {
			System.out.println("-1");
		} else {
			System.out.println(n-sum);
		}
		sc.close();

	}

} Main public static void main(String[] args) {
		// TODO 自動生成されたメソッド・スタブ
		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 val:a) {
			sum += val;
		}
		if (n-sum < 0) {
			System.out.println("-1");
		} else {
			System.out.println(n-sum);
		}
		sc.close();

	} main {
		// TODO 自動生成されたメソッド・スタブ
		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 val:a) {
			sum += val;
		}
		if (n-sum < 0) {
			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 []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 val:a) {
			sum += val;
		} int val a {
			sum += val;
		} {
			sum += val;
		} sum += val sum val if (n-sum < 0) {
			System.out.println("-1");
		} else {
			System.out.println(n-sum);
		} n-sum < 0 n-sum n sum 0 {
			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[] args args public class Main {

	public static void main(String[] args) {
		// TODO 自動生成されたメソッド・スタブ
		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 val:a) {
			sum += val;
		}
		if (n-sum < 0) {
			System.out.println("-1");
		} else {
			System.out.println(n-sum);
		}
		sc.close();

	}

} public class Main {

	public static void main(String[] args) {
		// TODO 自動生成されたメソッド・スタブ
		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 val:a) {
			sum += val;
		}
		if (n-sum < 0) {
			System.out.println("-1");
		} else {
			System.out.println(n-sum);
		}
		sc.close();

	}

} Main
import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		// TODO 自動生成されたメソッド・スタブ
		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 val:a) {
			sum += val;
		}
		if (n-sum < 0) {
			System.out.println("-1");
		} else {
			System.out.println(n-sum);
		}
		sc.close();

	}

}