#エラトステネスの篩
m=2*10**6
prime=[]
d=[-1]*(m+1)
for i in range(2,m+1):
  if d[i]==-1:
    d[i]=i
    prime.append(i)
    j=2
    while i*j<=m:
      d[i*j]=i
      j+=1
l,r=map(int,input().split())
ans=0
for i in prime:
  if i==2:
    if l<=2<=r:
      ans+=1
    continue
  if l<=i<=r:
    ans+=1
  if l<=(i+1)//2<r:
    ans+=1
    #print(i)
print(ans)