l = [0] * (10 ** 7 + 1)
l[1]= 1
for i in range(2,int((10 ** 7) ** (1/2)) + 1):
    if l[i] == 0:
        n = i  * 2
        cnt = 2
        while n <= 10 ** 7:
            l[n] = 1
            cnt += 1
            n = i * cnt
L,R = map(int,input().split())
ans = 0
for i in range(1,10 ** 7 + 1):
    if L<= i <= R:
        if l[i] == 0:
            ans += 1
    if l[i] == 0:
        if i & 1:
            if L <= (i-1)//2 and (i+1)//2 <= R:
                ans += 1
print(ans)