package main

import (
	"errors"
	"fmt"
)

var errCapacity = errors.New("invalid capacity or used count")

func freeSlots(capacity, used int) (int, error) {
	if capacity < 0 || used < 0 || used > capacity {
		return 0, errCapacity
	}
	return capacity - used, nil
}

func main() {
	free, err := freeSlots(12, 5)
	if err != nil {
		panic(err)
	}
	fmt.Println(free)
	_, err = freeSlots(4, 9)
	fmt.Println(errors.Is(err, errCapacity))
}