l,r = map(int,input().split())
M = 2*r+5
prime = [0]*M
prime[1] = 1
ans = 0
for i in range(2,M):
    if prime[i]:
        continue
    prime[i] = 2
    for j in range(i*2,M,i):
        prime[j] = 1
for i in range(l,r+1):
    if prime[i] == 2:
        ans += 1
    if i < r and prime[2*i+1] == 2:
        ans += 1
print(ans)