def Sieve_of_Eratosthenes(N,mode=False):
    """Nまでのエラトステネスの篩を実行

    N:自然数
    mode:False->素数のリスト,True->素数かどうかのリスト
    (False->[2,3,5,...],True->[0,0,1,1,0,1,...])
    """

    if N==0:
        return [0]

    T=[1]*(N+1)
    T[0]=T[1]=0

    for x in range(4,N+1,2): T[x]=0
    for x in range(9,N+1,3): T[x]=0

    a=5
    Flag=0
    while a*a<=N:
        if T[a]:
            b=a*a
            c=2*a
            while b<=N:
                T[b]=0
                b+=c
        a+=2+2*Flag
        Flag^=1

    if mode:
        return T
    else:
        return [k for k in range(N+1) if T[k]]
#==================================================
L,R=map(int,input().split())
P=Sieve_of_Eratosthenes(2*R,False)

X=0
for p in P:
    if L<=p<=R:
        X+=1

for p in P:
    if p==2:
        continue

    a=(p-1)//2
    b=a+1

    if L<=a<=R and L<=b<=R and a!=b:
        X+=1

print(X)