import sys
input = sys.stdin.readline
import math

x=3*10**6

L=math.floor(math.sqrt(x)) # 平方根を求める

Primelist=[i for i in range(x+1)]
Primelist[1]=0 # 1は素数でないので0にする.
 
for i in Primelist:
    if i>L:
        break
    if i==0:
        continue
    for j in range(2*i,x+1,i):
        Primelist[j]=0

Primes=[Primelist[j] for j in range(x+1) if Primelist[j]!=0]

S=set(Primes)

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

ANS=0
for i in range(L,R+1):
    if i in S:
        ANS+=1

for i in range(L,R):
    if i+i+1 in S:
        ANS+=1

print(ANS)