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