l,r=map(int,input().split())
c=2*r+1
prime=[True]*(c+1)
prime[0]=False;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
ans=0
for a in range(l,r+1):
    if a in primes:
        ans+=1
for a in range(l,r):
    if 2*a+1 in primes:
        ans+=1
print(ans)