#!/usr/bin/env python3
# from typing import *

import sys
import io
import math
import collections
import decimal
import itertools
import bisect
import heapq


def input():
    return sys.stdin.readline()[:-1]


# sys.setrecursionlimit(1000000)

# _INPUT = """2 9
# """
# sys.stdin = io.StringIO(_INPUT)

INF = 10**10

def get_primes(N):
    a = [True] * (N + 1)
    l = []
    for n in range(2, N + 1):
        if a[n]:
            l.append(n)
            for i in range(n+n, N + 1, n):
                a[i] = False
    return l

L, R = map(int, input().split())

primes = get_primes(2*R+2)

i1 = bisect.bisect_left(primes, L)
j1 = bisect.bisect_left(primes, R+1)
ans = j1 - i1
if L < R:
    i2 = bisect.bisect_left(primes, 2*L+1)
    j2 = bisect.bisect_left(primes, 2*(R-1)+1+1)
    ans += j2 - i2
print(ans)