st: How to estimate exponential decay parameter?

["Song" <raravise@gmail.com>]

Hello,

My data shows a exponential decay pattern and I want to estimate exponential 
decay parameter. How can I estimate 'd' in the following model using Stata?

Sales(at t) = M*exp(-d*t)

M: initial sales
t: time
d: decay parameter

Thank you for your help.
Reo. 
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