import java.util.*;
public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int a=sc.nextInt();
		int b=sc.nextInt();
		for(int i=0;i<b;i++) {
			int k =sc.nextInt();
			a-=k;
		}
		if(a>=0) {
			System.out.println(a);
		}else {
			System.out.println(-1);
		}
		
		
	}

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

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int a=sc.nextInt();
		int b=sc.nextInt();
		for(int i=0;i<b;i++) {
			int k =sc.nextInt();
			a-=k;
		}
		if(a>=0) {
			System.out.println(a);
		}else {
			System.out.println(-1);
		}
		
		
	}

} Main public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int a=sc.nextInt();
		int b=sc.nextInt();
		for(int i=0;i<b;i++) {
			int k =sc.nextInt();
			a-=k;
		}
		if(a>=0) {
			System.out.println(a);
		}else {
			System.out.println(-1);
		}
		
		
	} main {
		Scanner sc = new Scanner(System.in);
		int a=sc.nextInt();
		int b=sc.nextInt();
		for(int i=0;i<b;i++) {
			int k =sc.nextInt();
			a-=k;
		}
		if(a>=0) {
			System.out.println(a);
		}else {
			System.out.println(-1);
		}
		
		
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int a=sc.nextInt(); a sc.nextInt() sc.nextInt sc int b=sc.nextInt(); b sc.nextInt() sc.nextInt sc for(int i=0;i<b;i++) {
			int k =sc.nextInt();
			a-=k;
		} int i=0; int i=0; i 0 i<b i b i++ i++ i {
			int k =sc.nextInt();
			a-=k;
		} {
			int k =sc.nextInt();
			a-=k;
		} int k =sc.nextInt(); k sc.nextInt() sc.nextInt sc a-=k a k if(a>=0) {
			System.out.println(a);
		}else {
			System.out.println(-1);
		} a>=0 a 0 {
			System.out.println(a);
		} System.out.println(a) System.out.println System.out System System.out a {
			System.out.println(-1);
		} System.out.println(-1) System.out.println System.out System System.out -1 1 String[] args args public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int a=sc.nextInt();
		int b=sc.nextInt();
		for(int i=0;i<b;i++) {
			int k =sc.nextInt();
			a-=k;
		}
		if(a>=0) {
			System.out.println(a);
		}else {
			System.out.println(-1);
		}
		
		
	}

} public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int a=sc.nextInt();
		int b=sc.nextInt();
		for(int i=0;i<b;i++) {
			int k =sc.nextInt();
			a-=k;
		}
		if(a>=0) {
			System.out.println(a);
		}else {
			System.out.println(-1);
		}
		
		
	}

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

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int a=sc.nextInt();
		int b=sc.nextInt();
		for(int i=0;i<b;i++) {
			int k =sc.nextInt();
			a-=k;
		}
		if(a>=0) {
			System.out.println(a);
		}else {
			System.out.println(-1);
		}
		
		
	}

}
