# import necessary libraries
import pandas as pd
from sklearn.metrics import mean_squared_error

# 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])

# use mean_squared_error() function
mse = mean_squared_error(y_true, y_pred)

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