import java.util.Scanner;

public class Main {

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

    public static void main(String[] args) {
        
        Scanner sc = new Scanner(System.in);
        
        int n = Integer.parseInt(sc.next());
        int m = Integer.parseInt(sc.next());
        int sum = 0;
        for(int i = 0; i < m; i++){
            sum += Integer.parseInt(sc.next());
        }
        
        if(n >= sum){
            System.out.println(n - sum);
        }else{
            System.out.println(-1);
        }
    }
} Main public static void main(String[] args) {
        
        Scanner sc = new Scanner(System.in);
        
        int n = Integer.parseInt(sc.next());
        int m = Integer.parseInt(sc.next());
        int sum = 0;
        for(int i = 0; i < m; i++){
            sum += Integer.parseInt(sc.next());
        }
        
        if(n >= sum){
            System.out.println(n - sum);
        }else{
            System.out.println(-1);
        }
    } main {
        
        Scanner sc = new Scanner(System.in);
        
        int n = Integer.parseInt(sc.next());
        int m = Integer.parseInt(sc.next());
        int sum = 0;
        for(int i = 0; i < m; i++){
            sum += Integer.parseInt(sc.next());
        }
        
        if(n >= sum){
            System.out.println(n - sum);
        }else{
            System.out.println(-1);
        }
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int n = Integer.parseInt(sc.next()); n Integer.parseInt(sc.next()) Integer.parseInt Integer sc.next() sc.next sc int m = Integer.parseInt(sc.next()); m Integer.parseInt(sc.next()) Integer.parseInt Integer sc.next() sc.next sc int sum = 0; sum 0 for(int i = 0; i < m; i++){
            sum += Integer.parseInt(sc.next());
        } int i = 0; int i = 0; i 0 i < m i m i++ i++ i {
            sum += Integer.parseInt(sc.next());
        } {
            sum += Integer.parseInt(sc.next());
        } sum += Integer.parseInt(sc.next()) sum Integer.parseInt(sc.next()) Integer.parseInt Integer sc.next() sc.next sc if(n >= sum){
            System.out.println(n - sum);
        }else{
            System.out.println(-1);
        } n >= sum n sum {
            System.out.println(n - sum);
        } System.out.println(n - sum) System.out.println System.out System System.out n - sum n sum {
            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 = Integer.parseInt(sc.next());
        int m = Integer.parseInt(sc.next());
        int sum = 0;
        for(int i = 0; i < m; i++){
            sum += Integer.parseInt(sc.next());
        }
        
        if(n >= sum){
            System.out.println(n - sum);
        }else{
            System.out.println(-1);
        }
    }
} public class Main {

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

public class Main {

    public static void main(String[] args) {
        
        Scanner sc = new Scanner(System.in);
        
        int n = Integer.parseInt(sc.next());
        int m = Integer.parseInt(sc.next());
        int sum = 0;
        for(int i = 0; i < m; i++){
            sum += Integer.parseInt(sc.next());
        }
        
        if(n >= sum){
            System.out.println(n - sum);
        }else{
            System.out.println(-1);
        }
    }
}