#yuki1657
l,r=map(int,input().split())
c=2*r+1
prime=[True]*(c+1)
prime[0]=prime[1]=False
primes=set()
for i in range(2,c+1):
 if prime[i]:
  primes.add(i)
  for j in range(2*i,c+1,i):
   prime[j]=False
res=0
for a in range(l,r+1):
 if a in primes:
  res+=1
for a in range(l,r):
 if 2*a+1 in primes:
  res+=1
print(res)