The cache is hidden somewhere in Aleksandrovsky park (Gorkovskaya metro station).
The coordinates lead you to the Monument to Architects and Town-planners of Saint Petersburg (photo #1).
There is an empty seat near the table. SIT DOWN, relax and imagine that YOU ARE AN ARCHITECT. Do it, seriously!
OK, now when you are sitting... Look at the guy on the right. Some serious dude, isn't he? The note with further instructions is hidden in his right ear.
Just kidding, leave the man alone :)
Now you must count the number of people sitting at the table. It's A. And the number of all architects around (do not count the muggle architects dangling around!). It's B.
The cache is hidden at N59 57.(A - 3)13 E030 19.(9 - B)23.
Be carefull, lots on muggles around.