When I solved this problem then understood that I don't know how humanly round down float to int in C++, because int(double) round down positive numbers but round up negative.
Now I use int(x+1000000000)-1000000000, but it is bad.
Any ideas ...

why don't you use floor() function?

Good idea. It is possible to use int(floor()).
Nowhere could find proper function. Thnks.