n,x,d = gets.chomp.split(" ").map(&:to_i)
i = 0
min = 0
count = 0
for i in 1..n
  min_after = (x+(x+d*(i-1)))*i/2
  max_after = ((x+d*(n-1))+(x+d*(n-i)))*i/2
  if max >= min_after
    min = max + d
  else
    min = min_after
  end
  max = max_after
  count += ((max - min)/d+1)
end

if x * (x+d*(n-1)) < 0
  p count
else
  p count+1
end
  