import java.util.*;
import java.lang.*;
 
public class Main {
	public static void main(String[] args) {
	Scanner sc = new Scanner(System.in);
	int n = sc.nextInt();
	int m = sc.nextInt();
	for(int i = 0; i < m; i++) {
		n -= sc.nextInt();
	}
	if(n >= 0) {
		System.out.println(n);
	}else{
		System.out.println("-1");
	}
	}
} import java.util.*; util.* java import java.lang.*; lang.* java public class Main {
	public static void main(String[] args) {
	Scanner sc = new Scanner(System.in);
	int n = sc.nextInt();
	int m = sc.nextInt();
	for(int i = 0; i < m; i++) {
		n -= sc.nextInt();
	}
	if(n >= 0) {
		System.out.println(n);
	}else{
		System.out.println("-1");
	}
	}
} Main public static void main(String[] args) {
	Scanner sc = new Scanner(System.in);
	int n = sc.nextInt();
	int m = sc.nextInt();
	for(int i = 0; i < m; i++) {
		n -= sc.nextInt();
	}
	if(n >= 0) {
		System.out.println(n);
	}else{
		System.out.println("-1");
	}
	} main {
	Scanner sc = new Scanner(System.in);
	int n = sc.nextInt();
	int m = sc.nextInt();
	for(int i = 0; i < m; i++) {
		n -= sc.nextInt();
	}
	if(n >= 0) {
		System.out.println(n);
	}else{
		System.out.println("-1");
	}
	} 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 for(int i = 0; i < m; i++) {
		n -= sc.nextInt();
	} int i = 0; int i = 0; i 0 i < m i m i++ i++ i {
		n -= sc.nextInt();
	} {
		n -= sc.nextInt();
	} n -= sc.nextInt() n sc.nextInt() sc.nextInt sc if(n >= 0) {
		System.out.println(n);
	}else{
		System.out.println("-1");
	} n >= 0 n 0 {
		System.out.println(n);
	} System.out.println(n) System.out.println System.out System System.out n {
		System.out.println("-1");
	} System.out.println("-1") System.out.println System.out System System.out "-1" 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();
	for(int i = 0; i < m; i++) {
		n -= sc.nextInt();
	}
	if(n >= 0) {
		System.out.println(n);
	}else{
		System.out.println("-1");
	}
	}
} public class Main {
	public static void main(String[] args) {
	Scanner sc = new Scanner(System.in);
	int n = sc.nextInt();
	int m = sc.nextInt();
	for(int i = 0; i < m; i++) {
		n -= sc.nextInt();
	}
	if(n >= 0) {
		System.out.println(n);
	}else{
		System.out.println("-1");
	}
	}
} Main
import java.util.*;
import java.lang.*;
 
public class Main {
	public static void main(String[] args) {
	Scanner sc = new Scanner(System.in);
	int n = sc.nextInt();
	int m = sc.nextInt();
	for(int i = 0; i < m; i++) {
		n -= sc.nextInt();
	}
	if(n >= 0) {
		System.out.println(n);
	}else{
		System.out.println("-1");
	}
	}
}