from random import randint

# l=int(input())
# r=int(input())
# a=int(input())

def solve_slow(L, r, a):
  raz=r-L
  L=0
  if raz<a:
    return 0
  elif raz==a:
    return 1
  else:
    for i in range(1,(raz//a)):
      L+=(i*a)
    L+=(raz//a)
    if (raz%a)!=0:
      L+=(raz//a)*(raz%a)
    return L

def solve(l, r, a):
  raz=r-l
  otvet=c=0
  if raz<a:
      return 0
  elif raz==a:
      return 1
  else:
      c=(raz//a)-1
      otvet=(int(((1+c)/2*c)) * a) + (raz//a) + ((raz//a)*(raz%a))
      return otvet

# print( solve_slow(l,r,a), solve(l, r, a) )

MAX_VAL = 10000000
for test in range(10000):
  l = randint(1, MAX_VAL)
  r = randint(l + 1, 2 * MAX_VAL)
  a = randint(1, 10)

  if solve_slow(l,r,a) != solve(l, r, a):
    print(l, r, a)
    break