About Laptop's backpack wetting

my laptap and notebook  get wetting !!!!!

I often used backpack to carry laptop and note book.

because A heavy rain , my backpack and laptop all get wet yesterday.

i want to avoid such a situation so i search waterproof backpack.

But waterproof backpack is too expensive.

Did you have the experience like me ?

And how did you fix this problem??  Can you teach me???