#エラトステネスの篩Trueなら素数
#勝手に0のFalseを追加した版
def sieve(n):
    is_prime = [True for _ in range(n+1)]
    is_prime[0] = False

    for i in range(2, n+1):
        if is_prime[i-1]:
            j = i * i
            while j <= n:
                is_prime[j-1] = False
                j += i
    table = [ i for i in range(1, n+1) if is_prime[i-1]]
    return [False]+is_prime, table
A,T =  sieve(10**6*2+100)
L,R = map(int, input().split())
cnt = int(A[R])
for i in range(L,R):
    if A[i]:
        cnt+=1
    if A[2*i+1]:
        cnt+=1

print(cnt)