def primeset(N): #N以下の素数をsetで求める.エラトステネスの篩O(√Nlog(N))
    lsx = [1]*(N+1)
    for i in range(2,int(-(-N**0.5//1))+1):
        if lsx[i] == 1:
            for j in range(i,N//i+1):
                lsx[j*i] = 0
    setprime = set()
    for i in range(2,N+1):
        if lsx[i] == 1:
            setprime.add(i)
    return setprime

L,R = map(int,input().split())
# (A+B)*(B-A+1) = 2*P
# A+B = 2 ^ B-A+1 = P -> A=B=1 P=1 x
# A+B = P ^ B-A+1 = 2 -> B=A+1 P=2*A+1 o
# A+B = 2*P ^ B-A+1 = 1 -> B=A P=A=B o
# A+B = 1 ^ B-A+1 = 2*P -> x
setp = primeset(2*10**6)
ans = 0
for i in range(L,R+1):
    if i in setp:
        ans += 1
    if (i+1 <= R) and (2*i + 1) in setp:
        ans += 1
print(ans)