Help with sample size for overall survival endpoint

Hi everyone!
Please help to calculate sample size for primary endpoint - overall survival after 36 months. It is a superiority hypothesis: H0: ϒ ≥ δ and H1: ϒ < δ, where ϒ- hazard ratio, δ- superiority margin. Type I error: 2.5%, Type II error: 20% (β=0.2), expected hazard ratio for tested and reference groups: ϒ=0.7 and superiority margin for hazard ratio is 1.0. Accrual period: 36 months, follow-up period: 36 months or derive a formula to reject a null hypothesis.

Thanks a lot!
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Less is more. Stay pure. Stay poor.
No interim analysis, just a single primary analysis at 36 months after the last person is enrolled?