import java.io.BufferedReader;
import java.io.InputStreamReader;
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 total =0;
		
		for(int i=0; i<m; i++) {
			total += sc.nextInt();
		}
		
		if ( n >= total) {
			System.out.println(n-total);
		}else {
			System.out.println(-1);
		}
	}
} import java.io.BufferedReader; BufferedReader java.io import java.io.InputStreamReader; InputStreamReader java.io 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 total =0;
		
		for(int i=0; i<m; i++) {
			total += sc.nextInt();
		}
		
		if ( n >= total) {
			System.out.println(n-total);
		}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();

		int total =0;
		
		for(int i=0; i<m; i++) {
			total += sc.nextInt();
		}
		
		if ( n >= total) {
			System.out.println(n-total);
		}else {
			System.out.println(-1);
		}
	} main {
		
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int m = sc.nextInt();

		int total =0;
		
		for(int i=0; i<m; i++) {
			total += sc.nextInt();
		}
		
		if ( n >= total) {
			System.out.println(n-total);
		}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 int total =0; total 0 for(int i=0; i<m; i++) {
			total += sc.nextInt();
		} int i=0; int i=0; i 0 i<m i m i++ i++ i {
			total += sc.nextInt();
		} {
			total += sc.nextInt();
		} total += sc.nextInt() total sc.nextInt() sc.nextInt sc if ( n >= total) {
			System.out.println(n-total);
		}else {
			System.out.println(-1);
		} n >= total n total {
			System.out.println(n-total);
		} System.out.println(n-total) System.out.println System.out System System.out n-total n total {
			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 n = sc.nextInt();
		int m = sc.nextInt();

		int total =0;
		
		for(int i=0; i<m; i++) {
			total += sc.nextInt();
		}
		
		if ( n >= total) {
			System.out.println(n-total);
		}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();

		int total =0;
		
		for(int i=0; i<m; i++) {
			total += sc.nextInt();
		}
		
		if ( n >= total) {
			System.out.println(n-total);
		}else {
			System.out.println(-1);
		}
	}
} Main
import java.io.BufferedReader;
import java.io.InputStreamReader;
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 total =0;
		
		for(int i=0; i<m; i++) {
			total += sc.nextInt();
		}
		
		if ( n >= total) {
			System.out.println(n-total);
		}else {
			System.out.println(-1);
		}
	}
}
