from bisect import bisect_left
def eratosthenes(n):
    Primes = []
    Is_prime = [True for _ in range(n+1)]
    Is_prime[0] = False
    Is_prime[1] = False
    for p in range(0, n+1):
        if not Is_prime[p]:
            continue
        Primes.append(p)
        for i in range(p*p, n+1, p):
            Is_prime[i] = False
    return Primes
l, r = map(int, input().split())
P = eratosthenes(r)[bisect_left(eratosthenes(r), l):]
Q = eratosthenes(2*r-1)[bisect_left(eratosthenes(2*r-1), 2*l+1):]
print(len(P)+len(Q))