from math import *


def Sieve(n):
    lst = [True] * (n + 1)
    S = set()
    for i in range(2, ceil(sqrt(n)) + 1):
        if lst[i]:
            for j in range(2 * i, n + 1, i):
                lst[j] = False
    for i in range(2, n + 1):
        if lst[i]:
            S.add(i)
    return S


L, R = map(int, input().split())
S = Sieve(2020202)
ans = 0
for i in range(L, R + 1):
    if i in S:
        ans += 1
        
for i in range(L, R):
    if 2 * i + 1 in S:
        ans += 1
print(ans)