import numpy as np
import sys
input2 = sys.stdin.read

# A: Anti-clockwise, B: Clockwise, V: Values
n, c = map(int, input().split())
AV = np.array(list(map(int, input2().rsplit())), dtype=np.int64)
A, V = AV[::2], AV[1::2]
B = (c - A)[::-1]

XA = np.cumsum(V) - A
XB = np.cumsum(V[::-1]) - B
RXA, RXB = XA - A, XB - B    # Cumulative Values returning to Start
NMA, NMB = XA.max(), XB.max()


def ind(lis, maxi):
    for i, x in enumerate(lis):
        if x == maxi:
            return i


MRA2, MRB2 = max(RXA[:-1], default=0), max(RXB[:-1], default=0)
mrai = ind(RXA, MRA2)
mrbi = ind(RXB, MRB2)

if not mrai:
    mrai = 0
if not mrbi:
    mrbi = 0

MA2 = max(0, max(XA[:n - 1 - mrbi], default=0))
MB2 = max(0, max(XB[:n - 1 - mrai], default=0))
res = max(NMA, NMB, MA2 + MRB2, MB2 + MRA2)


for k in range(-1, -10 if n > 10 else -2, -1):
    MA3, MB3 = max(XA[:k], default=0), max(XB[:k], default=0)
    mai3 = ind(XA, MA3)
    mbi3 = ind(XB, MB3)

    MRA3 = max(0, max(RXA[:n - 1 - mbi3], default=0))
    MRB3 = max(0, max(RXB[:n - 1 - mai3], default=0))

    res = max(res, MA3 + MRB3, MB3 + MRA3)
print(res)
