L,R = map(int,input().split())

MAX = 2*(10**6) + 10
sieve = [i for i in range(MAX)]
for i in range(2,MAX):
    if sieve[i] == i:
        for j in range(i,MAX,i):
            sieve[j] = min(sieve[j],sieve[i])
ans = 0
    
for i in range(2*L,2*R+1):
    if sieve[i] == i:
        if 2*L<=i-1 and i+1 <= 2*R:
            ans += 1
for i in range(max(2,L),R+1):
    if sieve[i] == i:
        ans += 1
print(ans)