def sieve_eratosthenes(n):
    primes = [0, 1] * (n // 2 + 1)
    if n % 2 == 0:
        primes.pop()
    primes[1] = 0
    primes[2] = 1
    for p in range(3, int(n ** 0.5) + 1, 2):
        if primes[p]:
            for q in range(p * p, n + 1, 2 * p):
                primes[q] = 0
    return primes

L,R = map(int,input().split())
era = sieve_eratosthenes(2*R+1)
ans = 0
for i in range(L,R):
  num = 2*i+1
  if era[num] == 1:
    ans += 1
for i in range(L,R+1):
  if era[i] == 1:
    ans += 1

print(ans)