def primes(n):
    is_prime = [True] * (n + 1)
    is_prime[0] = False
    is_prime[1] = False
    for i in range(2, int(n**0.5) + 1):
        if not is_prime[i]:
            continue
        for j in range(i * 2, n + 1, i):
            is_prime[j] = False
    return is_prime

l, r = map(int, input().split())
pr = primes(r * 2)
ans = 0
for i in range(l, r + 1):
    if i < r:
        if pr[i * 2 + 1]:
            ans += 1
    if pr[i]:
        ans += 1
print(ans)