import sys
import os

sys.setrecursionlimit(100000)
is_local = "TERM_PROGRAM" in os.environ
input = sys.stdin.readline
INF = float('inf')
MOD = 998244353

def debug(*args, **kwargs):
    if is_local:
        print(*args, **kwargs)
def I(): return input().rstrip()
def IS(): return input().split()
def II(): return int(input())
def IIS(): return map(int, input().split())
def LIIS(): return list(map(int, input().split()))

def main():
    n, d = IIS()
    loss = []
    gain = []
    for _ in range(n):
        p, q = IIS()
        loss.append(p)
        gain.append(q)

    m = -INF
    am = None
    amp = None
    for i in range(n):
        for j in range(n):
            if i == j:
                continue
            p0 = -loss[i]
            p1 = -loss[i] + gain[i] - loss[j]
            dx = -loss[i] + gain[i] -loss[j] + gain[j]
            if dx < 0:
                p0 = p0 + (d//2 - 1) * dx
                p1 = p1 + (d//2 - 1) * dx

            if min(p0, p1) > m:
                m = min(p0, p1)
                amp = (p0, p1)
                am = (i, j)
    print(m)


if __name__ == "__main__":
    main()