import sys
input = sys.stdin.readline
L, R = map(int, input().split())

class sieve:
  def __init__(self, n):
    self.n = n
    self.sv = [1] * (n + 1)
    self.sv[0] = 0
    self.sv[1] = 0
    self.minp = [n] * (n + 1)
    for i in range(2, n + 1):
      if self.sv[i]:
        self.minp[i] = i
        for j in range(i * 2, n + 1, i):
          self.sv[j] = 0
          self.minp[j] = min(self.minp[j], i)
  def isprime(self, x):
    if x > self.n:
      return False
    return self.sv[x] == 1
  def factorize(self, x):
    if self.sv[x]: return [(x, 1)]
    res = []
    while x > 1:
      p = self.minp[x]
      c = 0
      while x % p == 0:
        x //= p
        c += 1
      res.append((p, c))
    return res
  def modlcm(self, a, mod):
    res = [0] * (self.n + 1)
    ex = set()
    for i in range(len(a)):
      f = self.factorize(a[i])
      for j in f:
        if j > self.n:
          ex.add(j)
          continue
        res[j] = max(f.count(j), res[j])
    rres = 1
    for i in range(self.n + 1):
      if res[i] != 0:
        rres *= pow(i, res[i], mod)
        rres %= mod
    for i in ex:
      rres *= i
      rres %= mod
    return rres

sv = sieve(R * 2)

res = 0
for p in range(R * 2 + 1):
  if sv.isprime(p):
    if p in range(L, R + 1): res += 1
    x = (p - 1) // 2
    y = -((1 - p) // 2)
    if x in range(L, R + 1) and y in range(L, R + 1): res += 1
print(res)