Project Euler #2 Even Fibonacci numbers

#Each new term in the Fibonacci sequence is generated by adding the previous two terms. By starting with 1 and 2, #the first 10 terms will be:

#1, 2, 3, 5, 8, 13, 21, 34, 55, 89, …

#Find the sum of all the even-valued terms in the sequence which do not exceed four million.

#Analysis
#It can be demonstrated that every third term of the sequence is even.

limit = 4000000
x, y, sum = 1, 1, 0

while (sum < limit):
    sum += (x + y)
    x, y = x + 2*y, 2*x + 3*y

print sum