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
    if l <= i <= r:
        ans += 1
    elif l <= 2*i+1 < r:
        ans += 1
    for j in range(i,M,i):
        prime[j] = 1
print(ans)