l, r = map(int, input().split()) prime = [0 for i in range(r*2)] for i in range(2,r*2): if prime[i] == 0: kari = i*2 while kari < r*2: prime[kari] = 1 kari += i ans = 0 prime[1] = 1 for i in range(l,r+1): if prime[i] == 0: ans += 1 if l < r: for i in range(l*2+1,r*2): if prime[i] == 0: ans += 1 print("{}".format(ans))