l,r=map(int,input().split())

###########
prime_num=2*r
###########


min_prime=[-1]*(prime_num+1) #2以上の自然数に対して最小の素因数を表す
min_prime[0]=0
min_prime[1]=1
i=2
prime=[]
while i<=prime_num:
    if min_prime[i]==-1:
        min_prime[i]=i
        prime.append(i)
    for j in prime:
        if i*j>prime_num or j>min_prime[i]:break
        min_prime[j*i]=j
    i+=1

def prifac(n):
    res={}
    x=n
    while x>1:
        p=min_prime[x]
        if p in res:res[p]+=1
        else:res[p]=1
        x//=p
    return res


ans=0
for p in prime:
    if l<=p<=r:ans+=1
    if p==2:continue
    a=(p-1)//2
    b=(p+1)//2
    if l<=a<=b<=r:
        ans+=1

print(ans)