import numpy as np


def mse_loss(y_true, y_pred):
    # y_true and y_pred are numpy arrays of the same length
    return ((y_true - y_pred) ** 2).mean()

# Code to test mse_loss function
# y_true = np.array([1, 0, 0, 1])
# y_pred = np.array([0, 0, 0, 0])
# print(mse_loss(y_true, y_pred))  # 0.5
