from collections import defaultdict
#qweqweqwe
# I
N, M = map(int, input().split())
cup = defaultdict(int)

for _ in range(N):
    x, s = map(int, input().split())
    cup[x] += s

# summa dalās ar M
pairs = 0

for x in cup:
    if x >= M - x and cup[x] > 0:
        if x == M - x:
            pairs += cup[x] // 2
            cup[x] %= 2
        else:
            if M - x in cup:
                pairs += min(cup[x], cup[M - x])
                cup[x] -= min(cup[x], cup[M - x])
                cup[M - x] -= min(cup[x], cup[M - x])


print(pairs)