John and Julia are repainting the outside of their house. They each go out to buy paint. John buys enough paint to cover $\frac{4}{9}$ of their house and Julia buys enough to paint $\frac{5}{6}$ of their house. If it takes $1$ can of paint to paint $\frac{1}{18}$ of their house and they don't want to have any paint left over after they finish painting, how many cans of paint should they return to the store?

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