import sys

int1 = lambda x: int(x)-1
p2D = lambda x: print(*x, sep="\n")
def II(): return int(sys.stdin.readline())
def LI(): return list(map(int, sys.stdin.readline().split()))
def LI1(): return list(map(int1, sys.stdin.readline().split()))
def LLI(rows_number): return [LI() for _ in range(rows_number)]
def LLI1(rows_number): return [LI1() for _ in range(rows_number)]
def SI(): return sys.stdin.readline().rstrip()
# dij = [(0, 1), (-1, 0), (0, -1), (1, 0)]
dij = [(0, 1), (-1, 0), (0, -1), (1, 0), (1, 1), (1, -1), (-1, 1), (-1, -1)]
inf = 10**16
# md = 998244353
md = 10**9+7

from functools import lru_cache

class Sieve:
    def __init__(self, n):
        self.plist = [2]  # n以下の素数のリスト
        min_prime_factor = [2, 0] * (n // 2 + 1)
        for x in range(3, n + 1, 2):
            if min_prime_factor[x] == 0:
                min_prime_factor[x] = x
                self.plist.append(x)
                if x ** 2 > n: continue
                for y in range(x ** 2, n + 1, 2 * x):
                    if min_prime_factor[y] == 0:
                        min_prime_factor[y] = x
        self.min_prime_factor = min_prime_factor

    def isprime(self, x):
        return self.min_prime_factor[x] == x

    # これが素因数分解(prime factorization)
    def pfct(self, x):
        pp, ee = [], []
        while x > 1:
            mpf = self.min_prime_factor[x]
            if pp and mpf == pp[-1]:
                ee[-1] += 1
            else:
                pp.append(mpf)
                ee.append(1)
            x //= mpf
        return [(p, e) for p, e in zip(pp, ee)]

@lru_cache()
def isp(a):
    for p in sv.plist:
        if p**2>a:return True
        if a%p==0:return False

mx=10**6+5

sv=Sieve(mx)
l,r=LI()

ans=0
for a in range(l,r+1):
    if sv.isprime(a):ans+=1
    if a==r:break
    s=2*a+1
    if s<mx:
        if sv.isprime(s):ans+=1
    else:
        if isp(s):ans+=1

print(ans)