1000-digit Fibonacci number : Problem 25

1000-digit Fibonacci number : Problem 25

The Fibonacci sequence is defined by the recurrence relation:

F_n = F_n−1 + F_n−2, where F₁ = 1 and F₂ = 1.

Hence the first 12 terms will be:

F₁ = 1
F₂ = 1
F₃ = 2
F₄ = 3
F₅ = 5
F₆ = 8
F₇ = 13
F₈ = 21
F₉ = 34
F₁₀ = 55
F₁₁ = 89
F₁₂ = 144

The 12th term, F₁₂, is the first term to contain three digits.
What is the first term in the Fibonacci sequence to contain 1000 digits?

Problem Source: Euler Project

Python Code

def ThousandDigitFiboNumbers():
    a = 1
    b = 1
    nextSum = 0
    totalSum = 2
    while True:
        nextSum = a+b
        a = b
        b = nextSum
        totalSum = totalSum + 1
        if len(str(nextSum)) == 1000:
            break
    print "first term in the Fibonacci sequence to contain 1000 digits:",totalSum       
ThousandDigitFiboNumbers()