How Many Letters Appear an Even Number of Times?

The wizard's spell only works if enough letters come in pairs. 🪄

You get a lowercase word. For each DIFFERENT letter in it, count its appearances; print how many different letters appear an even number of times.

Example:

Input: word = "hello" Output: 1

Counting each different letter gives h=1, e=1, l=2, o=1 — only "l" has an even count.