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

C = 2 * R  + 2
dat = [0] * C
prime = set()

i = 2
while i < C:
    if dat[i] == 0:
        primt.add(i)
        for j in range(i * 2,C,i):
            dat[j] = 1
    i += 1
ans = 0
for A in range(L,R + 1):
    if A in prime:
        ans += 1
for A in range(L,R):
    if 2 * A + 1 in prime:
        ans += 1
print(ans)