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