def accum_dp(xs: list, f, op, e, init: dict):
    dp = init.copy()
    for x in xs:
        pp = {}
        dp, pp = pp, dp
        for fm_key, fm_val in pp.items():
            for to_key, to_val in f(fm_key, fm_val, x):
                dp[to_key] = op(dp.get(to_key, e), to_val)

    return dp


INF = 1 << 62
N, D = map(int, input().split())

tasks = []
for _ in range(N):
    T, K = map(int, input().split())
    tasks.append((T, K))


def f(k, v, x):
    # k : 東京 or 京都

    a, b = x
    match k:
        case 0:
            yield 0, v+a
            yield 1, v+b-D

        case 1:
            yield 0, v+a-D
            yield 1, v+b

        case _:
            assert False


init = {0: 0}
dp = accum_dp(tasks, f, max, -INF, init)
ans = max(dp.values())
print(ans)