from bisect import bisect_left
def eratosthenes(s, t):
        Prime = []
        Data = [i+1 for i in range(1, t)]
        limit = t**0.5
        while True:
            p = Data[0]
            if p > limit:
                Prime += Data
                Prime = Prime[bisect_left(Prime, s):]
                return Prime
            Prime.append(p)
            Data = [e for e in Data if e%p != 0]
l, r = map(int, input().split())
P = eratosthenes(l, r)
P += eratosthenes(2*l+1, 2*r+1)
print(len(P))