L,R = map(int,input().split())
INFTY = 2*R
P = list(range(INFTY+1))
for i in range(2,INFTY+1):
    if i*i>INFTY:break
    for j in range(i*i,INFTY+1,i):
        P[j] = P[i]
Q = set()
for i in range(2,INFTY+1):
    if P[i]==i:
        Q.add(i)
ans = 0
for i in range(L,R+1):
    if i in Q:
        ans += 1
for i in range(L,R):
    if 2*i+1 in Q:
        ans += 1
print(ans)