On test
975461058253082170
 my program give answer
987654321.2344
And Standard Windows Calculator give answer
987654321,23445000902549242897908, e.g.
987654321.2345!
So, my AC program give wrong answer!
I used Double type and standart sqrt.

Bullshit.

Where in problem statement said that you have to round answer? 987654321.2344 is correct answer because 2344 are first four exact digits after comma in decimal record of sqrt(975461058253082170).