import functools

from flask import Flask, Response
app = Flask(__name__)

@functools.lru_cache(maxsize=None)
def fibonacci(i):
    a, b = 0, 1
    for j in range(i):
        a, b = b, b+a

    return b

@app.route('/fib/<number>')
def render_fib(number):
    try:
        num = int(number)
    except Exception:
        return Response("400 Bad Request", status=400)

    if num > 500000:
        return Response("403 Forbidden", status=403)

    resp = str(fibonacci(num)) + "\n"
    return Response(resp, mimetype='text/plain')