#int(input())
#map(int, input().split())
#list(map(int, input().split()))

L, R = map(int, input().split())

p = [-1] * (2*R)

for i in range(2, R+1):
    if p[i] != -1:
        continue
    for j in range(2, R+1):
        if i*j >= 2*R:
            break
        p[i*j] = i
p[1] = 1

ans = 0
for i in range(L, R):
    if p[i] == -1:
        ans += 1
    if p[2*i+1] == -1:
        ans += 1

if p[R] == -1:
    ans += 1

print(ans)