def proba_drifting(x):
    """
    gives more weight to outliers -- i.e. close to 0 and 1
    the polynomial was constructed with the following constraints:
    • f(0) = 0
    • f(0.5) = 0.5
    • f(1) = 1
    • f'(0) = 0
    • f'(0.5) = 1
    • f'(1) = 0
    """
    return 10 * x**3 - 15 * x**4 + 6 * x**5