import math

#Consts
v = 1 # velocity
z = 1 # pos in z

i = range(-1000,1000)
R = []
for j in i:
    R.append(math.sqrt((z-2*j))**2)

s = 0
for Ri in R:
    s += 1/Ri * math.exp(-v/2 * Ri)