# 素数の洗い出し
limit = 10**7

distinct_prime_factor_count = [0]*(limit+1)
primes = []
for i in range(2, limit+1):
    if distinct_prime_factor_count[i] == 0:
        primes.append(i)
        for num in range(i, limit+1, i):
            distinct_prime_factor_count[num] += 1

primes_set = set(primes)

L, R = map(int, input().split())

ans = 0
for a in range(L, R + 1):
    # a == b のパターン
    # a 自体が素数か
    if a in primes_set:
        ans += 1

    # a+1 == b のパターン
    if 2*a + 1 in primes_set:
        ans += 1
print(ans)