Question on formula for doing Randbetween If value is <20

How would you create a rand between to only appear if the sum of the four previous Rand between is <20 then i need to create another rand between .i all ready simulated four columns randomly rolling a twenty sided die and I already have added the sum of each row. How do I simulate to roll an additional two twenty sided die if the value of the four twenty sided dice are less the 20.

Thank you very much that helped out a lot.