import sys
read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines
sys.setrecursionlimit(10 ** 7)

a, b = map(int, readline().split())
print(sum(1 if (a + b + i) % 3 == 0 else 0 for i in range(a, b + 1)))