def F(N):
    """
    Fibonacci Sequence.
    F(n + 1) = F(n) + F(n - 1)
    """
    a = 0
    b = 1
    F = [a, b]
    for j in range(1, N + 1):
        c = a + b
        a = b
        b = c
        F.append(c)
    
    return F[N]