import math


def _negative_exp (x : int, alpha : float) -> float:
    """
    This method return the negative exponential according to equation
                        
                        f(x) = e^(-alpha*x)
    
    :param x: The input.
    :param alpha: The parameter of the exponential (the higher is alpha, the
                    faster is the decrease).
    :return: The output f(x).
    
    """
    return math.exp(-alpha*x)