import sys
input = lambda :sys.stdin.readline()[:-1]
ni = lambda :int(input())
na = lambda :list(map(int,input().split()))
sys.setrecursionlimit(10**7)
yes = lambda :print("yes");Yes = lambda :print("Yes");YES = lambda : print("YES")
no = lambda :print("no");No = lambda :print("No");NO = lambda : print("NO")
#######################################################################

L,R = na()

m = 2*10**6+2
#m = 100

a = [1 for i in range(m)]
a[0] = 0
a[1] = 0
p = []
for i in range(2,m):
    if not a[i]:
        continue
    p.append(i)
    j = i*2
    while j<m:
        a[j] = 0
        j+=i

ans = 0
for A in range(L,R+1):
    if a[A]:
        ans+=1
    if a[2*A+1] and A+1<=R:
        ans+=1
    #print(A,ans)
print(ans)