Sudoku Grand Prix 2015’s second round of competition opened up online Friday morning, pitting solvers from around the world against some diabolical Sudoku puzzles.

Set to run through Sunday, Sudoku GP’s second round features 13 puzzles that includes several Classic Sudoku, along with variants of the form like Nonconsecutive, Diagonal, and Star Product Sudoku. Each of the Sudoku were constructed by Russian designer Andrey Bogdanov, a Russian puzzle champion and contributor to Logic Masters India. The WPF has provided examples in an instruction booklet to give competitors an idea of the challenges that they will be facing.

Rounds of the tournament last 90 minutes, with only the six highest scores of seven contributing to each puzzler’s final total. The competition is run by the World Puzzle Federation and has Russia acting as host.

Entry into the Sudoku GP is free and doesn’t require having participated in the previous round of the tournament to solve. The only requirement is that all puzzlers must register for a free account to the Grand Prix website, which allows officials to keep track of the points earned throughout the competition.

Tiit Vunk of Estonia currently holds first in the GP, having taken a large lead ahead of the pack with second place finisher from the Czech Republic, Jakub Ondroušek, both outpacing the rest by nearly 100 points.

Sudoku Grand Prix Round Schedule

1. Netherlands: Jan. 2 – 5
2. Russia: Jan. 30 – Feb. 2
3. India: Feb. 27 – March 2
4. Poland: March 27 – 30
5. Serbia: April 24 – 27
6. Czech Republic: May 22 – 25
7. Italy: June 19 – 22
8. U.K.: July 17 – 20

Source: Sudoku GP