A new Pizza takeaway, Pizza Pi, has opened in Gletrian.

At Pizza Pi they make Pizzas in three different sizes.

One of the pizzas the takeaway offers is the Pepperoni Pizza.

The number of slices of Pepperoni depends on the size of the pizza.

The relationship between the diameter of the pizza `D` and the number of slices of pepperoni `N` can be expressed by the formula `N = kD<sup>2</sup>`, where `k` is a constant.

Customers can have extra pepperoni on their pizza.

The number of slices on of pepperoni when a customer orders extra is given by the formula `N = <sup>1</sup>/<sub>5</sub>kD<sup>3</sup>`.

The takeaway introduce a personal pizza.

The personal pizza has a diameter such that the number of slices of pepperoni is the same whether a customer orders extra or not.

Find the diameter of a personal pizza.