from sys import stdin,setrecursionlimit#,set_int_max_str_digits
#import pypyjit
#pypyjit.set_param('max_unroll_recursion=-1')
setrecursionlimit(20000000) # これこどふぉだと無理
#set_int_max_str_digits(200010)
mod = 998244353 
ii = lambda :int(stdin.readline())
mi = lambda :map(int,stdin.readline().split())
li = lambda :list(mi())
gmi = lambda :map(lambda x: int(x) - 1, stdin.readline().split())
gi = lambda :list(map(lambda x: 0 if x == "." else 1,input())) # グリッド入力受け取り
py = lambda :print("Yes")
pn = lambda :print("No")
pf = lambda :print("First") 
ps = lambda :print("Second")
vec = [(1,0),(-1,0),(0,-1),(0,1)]
vec1 = [(1,0),(1,1),(0,1),(-1,1),(-1,0),(-1,-1),(0,-1),(1,-1)] #8方向
inf = 2*10**18
from collections import defaultdict

l,r = mi() 

N = r+1

p_data = [-1]*(N+1) # 素数なら 1、素数でないなら最大の素因数 
p_list = [] # 素数のリストを返す

for i in range(l,r+1):
    if p_data[i] == -1:
        p_data[i] = 1 
        p_list.append(i)
        for j in range(2*i,N+1,i):
            p_data[j] = i 

#print(p_list)
print(len(p_list)-1)