import math


def poisson(lmbda: float,
            x: int) -> float:
    """
    Calculates the Poisson pdf.

    Extended description of function.

    Parameters
    ----------
    lmbda: description
    x: description

    Returns
    -------
    The appropriate integer for that selection.

    """
    return math.exp(-1 * lmbda) * math.pow(lmbda, x)/math.factorial(x)