Sample size calculation for survival clinical trial

#1
Hi!

I need to calculate sample size for phase 3 clinical trila with following assumption:
Primary Endpoint: Overall Survival (OS) in 36 months;
Study Hypothesis: superiority (H0: Υ ≥ δ; H1: Υ < δ);
Type I error: 2.5% (α = 0.025);
Type II error: 20% (β = 0.2);
Expected HR for Test vs Comparator arms: Υ = 0.7;
Superiority margin: δ = 1.0;
Median time for Comparator arm: m0 = 13 months;
Accrual period: 36 months;
Follow-up period: 36 months;
Drop-out rate: 10%
No interim analyses
And assuming exponential survival distribution

Please, can anyone share what open sources/tools or libraries in R I can use to perform this calculation?

Thank in advance!