L,R=map(int, input().split())
ans=0
D=[1]*(2*10**6+10)
D[0]=0
D[1]=0
for i in range(2,10**6+1):
  cnt=2
  if D[i]==1:
    while i*cnt<=2*10**6+5:
      D[i*cnt]=0
      cnt+=1
ans=0
if D[R]==1:
  ans+=1
for l in range(L,R):
  if D[l]==1:
    ans+=1
  a=2*l+1
  if D[a]==1:
    ans+=1
print(ans)