import math

def sieve(n):
    ass = []
    is_prime = [True]*(n+1)
    is_prime[0] = False
    is_prime[1] = False

    for i in range(2, int(math.sqrt(n))+1):
        if not is_prime[i]:
            continue
        for j in range(i*2, n+1, i):
            is_prime[j] = False
    for i in range(n+1):
        if is_prime[i]:
            ass.append(i)
    return(ass)

P = sieve(2*10**6)

l, r = map(int, input().split())
ans = 0
for p in P:
    if p == 2:
        if l <= 2 <= r:
            ans += 1
    else:
        a = (p-1)//2
        b = (p+1)//2
        if l <= a and b <= r:
            ans += 1
        if l <= p <= r:
            ans += 1
print(ans)