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