Runner.rb

def runner( x, y, size )                                                                            
  if x == size && y == size                                                                         
    puts "Path found"                                                                               
  end                                                                                               
  unless x + 1 > size                                                                              
    runner( x + 1, y, size )                                                                      
  end                                                                                              
  unless y + 1 > size                                                                               
    runner( x, y + 1, size )                                                                        
  end                                                                                               
end                                                                                                 
runner( 0 , 0, 20 )