import java.util.*;
import java.lang.*;
import java.io.*;

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

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_days = 0;
        for (int i = 0; i<m;i++){
            sum_days += sc.nextInt();
        }
            int res = n - sum_days;        
        if(res<0){
            res = -1;
        }
        System.out.println(res);
    }
}