Series approximation of Pi in Python

# -*- coding: cp1252 -*-

from mpmath import *
import time

dps = input("Number of decimals to compute: ")
if dps < 11:
    print "Choosing 11 digits"
    dps = 11

mp.dps = dps
mp.dps=mp.dps+1

a = 1
x = 1
b = mpf(1/mp.sqrt(2))
c = 0.25
i = 0
y = 1
derivedbefore = mpf(0)
breakyn = False
run = True
#print "a =", a
#print "x =", x
#print "b =", b
#print "c =", c
#print "i =", i
#print "Derived before:", derivedbefore
start = time.clock()
print "Start:", start
print "------------------------------------------------------"
print "------------------------------------------------------"
while run == True:
    i = i+1
    y = mpf(a)
    a = mpf((a+b)/2)
    b = mpf(mp.sqrt(b*y))
    c = mpf(c-x*(mp.power(a-y,2)))
    x = mpf(2*x)
    derived = mpf(mp.power((a+b),2)/(4*c))
    if str(derived)[-10:-1] == str(derivedbefore)[-10:-1]:
          break
    print "Iteration:", i
    #print
    #print "Computed:", derived
    derivedbefore = derived
    print "------------------------------------------------------"
stop=time.clock()
print "------------------------------------------------------"
print "Pi computed to ", mp.dps-1, "decimals:"
print
print derived
duration=stop-start
print "------------------------------------------------------"
print "Time of execution: ", duration, "Seconds."
print "Happy Pi Day!"