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 sum=0;
        int[] arr=new int[m];
        for (int i = 0; i <arr.length ; i++) {
            arr[i]=sc.nextInt();
            sum=sum+arr[i];
        }
        if(sum<=n) {
            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=sc.nextInt();
        int m=sc.nextInt();
        int sum=0;
        int[] arr=new int[m];
        for (int i = 0; i <arr.length ; i++) {
            arr[i]=sc.nextInt();
            sum=sum+arr[i];
        }
        if(sum<=n) {
            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=sc.nextInt();
        int m=sc.nextInt();
        int sum=0;
        int[] arr=new int[m];
        for (int i = 0; i <arr.length ; i++) {
            arr[i]=sc.nextInt();
            sum=sum+arr[i];
        }
        if(sum<=n) {
            System.out.println(n - sum);
        }else {
            System.out.println(-1);
        }








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








    }
}



