How to write a code for summation in a formaula?

Dear all,

I would like to write a formula consist of summation.

G(i) = m + sigma (j=0 to i-1) G(j) * m

in each level to calculate new G we have to use previous calculated Gs. for example:
for G(3) we need (G(0)*m + G(1)*m + G(2)*m) which have calculated in previous levels. And m is a constant(18.01).

I would like to write a code for this loop (data numbers come from a column in a text file) But as I am beginner to python I dont know how to start. Do I have to use more than one loop? a loop for G(i) and a loop for sigma? what can be the exact form?

Thank you in advance for your help

I tried simple codes for summation but I have no idea for this problem.