Puzzle friend sent me.

[mookboi]

"So apparently this is given to kids who are about to enter PRIMARY school in Russia. I couldnt solve it....

8809 = 6
7111 = 0
2172 = 0
6666 = 4
1111 = 0
3213 = 0
7662 = 2
9312 = 1
0000 = 4
2222 = 0
3333 = 0
5555 = 0
8193 = 3
8096 = 5
7777 = 0
9999 = 4
7756 = 1
6855 = 3
9881 = 5
5531 = 0
2581 = ?"

Took me like 10 mins to figure out, and I went to Primary school in Russia... :\

Sorry if repost/wrong kinda content for forum - never posted here before.