# import necessary libraries
import pandas as pd

# define your series
y_true = pd.Series([1, 2, 3, 4, 5])
y_pred = pd.Series([1.5, 2.5, 3.5, 4.5, 5.5])

# using panda's mean()
mse = ((y_true - y_pred) ** 2).mean()

# print result
print("MSE:", mse)