2813 : 소수의 개수

def eratos(b):
    global prime
    prime[0]=1
    prime[1]=1
    for i in range(2,b+1):
        if(prime[i]>0) : continue
        for j in range(i*i,b+1,i):
            prime[j]=1

prime = [0 for i in range(2000000 +1)]

eratos(2000000)

n,m = map(int,input().split())

cnt=0
for i in range(n,m+1):
    if prime[i]==0:cnt+=1

print(cnt)