def prime_list(n):
	is_prime = [0] * (n + 1)
	is_prime[2] = 1
	for i in range(3, n+1, 2):
		is_prime[i] = 1
	for i in range(2, int(n ** 0.5) + 1):
		if is_prime[i]:
			for j in range(i * i, n + 1, 2 * i):
				is_prime[j] = 0
	d = []
	for j in range(n + 1):
		if is_prime[j] == 1:
			d.append(j)
	return d

l, r = map(int, input().split())

p = set(prime_list(2*r + 1))
	
ans = 0
for i in range(l, r):
	if i in p:
		ans += 1
	if 2*i + 1 in p:
		ans += 1
if r in p:
	ans += 1
print(ans)