# import sys
# #sys.setrecursionlimit(1000000)
# input = sys.stdin.readline
def mp():return map(int,input().split())
def lmp():return list(map(int,input().split()))
import math
import bisect
from copy import deepcopy as dc
from itertools import accumulate
from collections import Counter, defaultdict, deque
import itertools
def ceil(U,V):return (U+V-1)//V
def modf1(N,MOD):return (N-1)%MOD+1
inf = int(1e20)
mod = 998244353
def primes(n):
    prime = [True] * (n + 1)
    prime[0] = False
    prime[1] = False
    for i in range(2, n + 1):
        for j in range(i * 2, n + 1, i):
            prime[j] = False
    return [i for i in range(n + 1) if prime[i]]

p = primes(int(10**6)*3)
p = set(p)
l,r = mp()
ans = 0
for i in range(l,r):
    if i + i + 1 in p:
        ans += 1
for i in range(l,r+1):
    if i in p:
        ans += 1
print(ans)