while True:

try:

a, b = [int(i) for i in input().split()]

n = 0

for t in range(a, b+1):

check = True

for i in range(2, t):

if t%i == 0:

check = False

if check: n += 1

print(n)

except:

break

while True:

try:

a, b = [int(i) for i in input().split()]

n = 0

for t in range(a, b+1):

check = True

for i in range(2, t):

if t%i == 0:

check = False

if check: n += 1

print(n)

except:

break

建表