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

def eratosthenes_sieve(n):
    is_prime = []
    d = [-1]*(n+1)
    for i in range(2, n+1):
        if d[i] == -1:
            d[i] = i
            is_prime.append(i)
            j=2
            while i*j <= n:
                d[i*j] = i
                j += 1
    return is_prime

n = 10**6
is_prime = eratosthenes_sieve(n)
for i in range(L,R):
    if i in is_prime:
        count += 1
    if 2*i+1 in is_prime:
        count += 1
if R in is_prime:
    count += 1

print(count)