A Classic Puzzle

This problem can be solved by pre-school children in 5-10 minutes, by programmers - in 1 hour, by people with higher
education ... well, check it yourself! :)

8809=6
7111=0 
2172=0 
6666=4
1111=0 
3213=0 
7662=2
9313=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=?