Primes_MAX = 2 * 10 ** 6 + 100
Primes = []
Sieve = [0] * Primes_MAX
Sieve[0] = Sieve[1] = -1
for p in range(2, Primes_MAX):
    if not Sieve[p]:
        Primes.append(p)
        Sieve[p] = p
        for q in range(p * p, Primes_MAX, p):
            if not Sieve[q]:
                Sieve[q] = p

l, r = map(int, input().split())
ans = 0
p = 0
for i in range(l, r):
    x = i + i + 1
    while Primes[p] < x:
        p += 1
    ans += Primes[p] == x

for i in range(l, r + 1):
    ans += Sieve[i] == i

print(ans)