A forecasting method has produced the information in the chart below over the past five months. What is the mean squared error $($MSE$)?$

Actual

Forecast

Error

$4$

$3$

$1$

$6$

$4$

$2$

$5$

$6$

$-1$

$3$

$5$

$-2$

$7$

$3$

$4$