A challenge for preschool children ...

This problem (judging by the search in Google) has already flashed many times in blogs and forums. But, for me, and I hope the majority of habra people - became a discovery ...

When they told me about it, I killed ten minutes to find an answer, and gave up ... When the veil of secrecy was ajar, the elegance of the solution put me in a stupor and made me laugh for a long time.

:
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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 =?

I remind you: the average preschooler solves it in 5 minutes.

PS I will ask you not to use cheating, such as Google, and not to post an answer prematurely, so as not to save other habra people from the pleasure ...