import sys
input = sys.stdin.readline

MAX = 10**6 * 2
table = [True] * (MAX + 1)
table[0] = table[1] = False

for i in range(2, MAX + 1):
    if i * i > MAX:
        break
    if table[i] == False:
        continue
    for j in range(i*2, MAX+1, i):
        table[j] = False

ans = 0
L, R = map(int, input().split())
for a in range(L, R+1):
    if table[a]:
        ans += 1
for a in range(L, R):
    b = a + 1
    if table[a + b]:
        ans += 1
print(ans)