L,R = map(int, input().split())
count = 0

def eratosthenes_sieve(n):
    is_prime = [True]*(n+1)
    is_prime[0] = is_prime[1] = False
    for p in range(2, n+1):
        if is_prime[p]:
            for q in range(2*p,n+1,p):
                is_prime[q] = False
    return is_prime

n = 10**5
is_prime = eratosthenes_sieve(n)
for i in range(L,R):
    if is_prime[i] == True:
        count += 1
    if is_prime[2*i+1] == True:
        count += 1
if is_prime[R] == True:
    count += 1

print(count)