L,R=map(int,input().split())
P=set()
M=2100000
F=[0]*M
for i in range(2,M):
  if F[i]:
    continue
  P.add(i)
  for j in range(i*2,M,i):
    F[j]=1
ANS=0
for i in range(L,R+1):
  if i in P:
    ANS+=1
  if i==R:
    break
  if i*2+1 in P:
    ANS+=1
print(ANS)