In the United States, radio and television broadcast stations are assigned unique identifiers known as call signs. Call signs consist of $4$ letters.

The first is either K or W $($generally speaking, stations with a K call sign are west of the Mississippi River and stations with a W call sign are east of the river, though there are several exceptions to this rule$)$; the remaining $3$ letters can be anything.

Hint: Recall that there are $26$ letters in the alphabet

How many possible call signs are there under this system?

$($Use commas in your answer$)$