def sieve_of_eratosthenes(max_num):
    is_prime = [True] * (max_num + 1)
    p = 2
    while (p * p <= max_num):
        if (is_prime[p] == True):
            for i in range(p * p, max_num + 1, p):
                is_prime[i] = False
        p += 1
    prime_numbers = [p for p in range(2, max_num + 1) if is_prime[p]]
    return prime_numbers

def count_twin_primes(L, R):
    primes = sieve_of_eratosthenes(R)
    primes_in_range = [p for p in primes if p >= L]
    twin_prime_count = 0
    for i in range(len(primes_in_range) - 1):
        if primes_in_range[i + 1] - primes_in_range[i] == 2:
            twin_prime_count += 1
            
    return twin_prime_count

L, R = map(int, input().split())
result = count_twin_primes(L, R)
print(result)