For a particular Java assembler interface, it is known that the operand stack size has the following probability distribution function:

$0$

$1$

$2$

$3$

$4$

$\mathrm{.}25$

$\mathrm{.}20$

$\mathrm{.}18$

$\mathrm{.}17$

$\mathrm{.}20$

What is the variance of the stack size?