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