Calculating Fibonacci number

fibo.py

import sys

A, B, N = [int(i) for i in sys.stdin.readline().split(' ')]

a_n = A
a_n_1 = B

t = 2
while t < N:
	i = a_n
	a_n = a_n_1
	a_n_1 = a_n**2 + i
	t += 1

print a_n_1