import sys
input = sys.stdin.readline
N, D = map(int, input().split())
a = []
b = []
t = []
for i in range(N):
  p, q = map(int, input().split())
  if p <= q: a.append((p, q, i))
  else: b.append((p, q, i))
  t.append((p, q))

s = set()

if len(a) >= 1:
  a.sort(key = lambda x: (x[0], -x[1]))
  s.add(a[0][2])

if len(a) >= 2:
  a.sort(key = lambda x: (x[0], -x[1]))
  s.add(a[1][2])

if len(a) >= 1:
  a.sort(key = lambda x: (-x[1], x[0]))
  s.add(a[0][2])

if len(a) >= 2:
  a.sort(key = lambda x: (-x[1], x[0]))
  s.add(a[1][2])

if len(b) >= 1:
  b.sort(key = lambda x: -x[1])
  s.add(b[0][2])

if len(b) >= 2:
  b.sort(key = lambda x: -x[1])
  s.add(b[1][2])

if len(b) >= 1:
  b.sort(key = lambda x: (x[0], -x[1]))
  s.add(b[0][2])

if len(b) >= 2:
  b.sort(key = lambda x: (x[0], -x[1]))
  s.add(b[1][2])

res = -10 ** 18
for i in s:
  for j in s:
    if i == j: continue
    tt = 0
    tres = 0
    for k in range(D):
      if k & 1:
        tt -= t[i][0]
        tres = min(tres, tt)
        tt += t[i][1]
      else:
        tt -= t[j][0]
        tres = min(tres, tt)
        tt += t[j][1]
    res = max(res, tres)

for i in range(N):
  for j in range(N):
    if i == j: continue
    tt = t[i][1] - t[i][0]
    tt += t[j][1] - t[j][0]
    if tt >= 0:
      tres = 0
      tres = min(tres, -t[i][0])
      tres = min(tres, t[i][1] - t[i][0] - t[j][0])
      res = max(res, tres)

print(res)