Power analysis and Kaplan meier


I need your help to compute a power analysis for survival analysis (Kaplan meier) without historical data.
In my lab we are preparing a project in which we want to evaluate tumor free survival of two cohorts of mouse (one treated with A and one treated with B); because the experiment is new, we don't know the median tumor incidence time of the two groups.

We want to make sure to detect at least a difference in tumor incidence of 20% between the two groups (using less mouse as possible).
I fixed alpha=0.05 and power=0.8. How can I calculate the required sample size (using R or STATA)??
How can I calculate the hazard ratio??

Thank you in advance for your help!
.. and sorry for my english errors
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