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();
		}

		if(0 <= n - sum) {
			System.out.println(n - sum);
		}else {
			System.out.println(-1);
		}
		sc.close();
	}
	
} 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();
		}

		if(0 <= n - sum) {
			System.out.println(n - sum);
		}else {
			System.out.println(-1);
		}
		sc.close();
	}
	
} 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();
		}

		if(0 <= n - sum) {
			System.out.println(n - sum);
		}else {
			System.out.println(-1);
		}
		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(0 <= n - sum) {
			System.out.println(n - sum);
		}else {
			System.out.println(-1);
		}
		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(0 <= n - sum) {
			System.out.println(n - sum);
		}else {
			System.out.println(-1);
		} 0 <= n - sum 0 n - sum n sum {
			System.out.println(n - sum);
		} System.out.println(n - sum) System.out.println System.out System System.out n - sum n sum {
			System.out.println(-1);
		} System.out.println(-1) System.out.println System.out System System.out -1 1 sc.close() sc.close sc 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();
		}

		if(0 <= n - sum) {
			System.out.println(n - sum);
		}else {
			System.out.println(-1);
		}
		sc.close();
	}
	
} 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();
		}

		if(0 <= n - sum) {
			System.out.println(n - sum);
		}else {
			System.out.println(-1);
		}
		sc.close();
	}
	
} 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();
		}

		if(0 <= n - sum) {
			System.out.println(n - sum);
		}else {
			System.out.println(-1);
		}
		sc.close();
	}
	
}