from itertools import chain


def prime_set(N):
    """
    Nまでの素数のsetを返す
    """
    if N < 4:
        return ({}, {}, {2}, {2, 3})[N]
    Nsq = int(N ** 0.5 + 0.5) + 1
    primes = {2, 3} | set(chain(range(5, N + 1, 6), range(7, N + 1, 6)))
    for i in range(5, Nsq, 2):
        if i in primes:
            primes -= set(range(i * i, N + 1, i * 2))
    return primes


U = 2 * 10 ** 6 + 10
Prime = prime_set(U)

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