text: #http://codeforces.com/contest/546/problem/A
  #problem solved by Benegripe with python 2.7
  #!/usr/bin/python
  
  k,n,w = raw_input().split()
  cost = ( (int(k) + int(w)*int(k))*int(w) ) / 2
  if (cost > int(n)):
  	print (cost - int(n))
  else:
  	print "0"
  