import sys
input = sys.stdin.buffer.readline
sys.setrecursionlimit(10 ** 7)
inf = 10 ** 19


N, D = map(int, input().split())
PQ = tuple(tuple(map(int, input().split())) for _ in range(N))
ans = -inf

for i, (p, q) in enumerate(PQ):
    for j, (pp, qq) in enumerate(PQ):
        if i == j:
            continue
        if (q - p + qq - pp) >= 0:
            ans = max(ans, min(-p, -p + q - pp))
        else:
            if D % 2 == 0:
                M = D // 2
                cnt = M * (q - p + qq - pp)
                ans = max(ans, min(cnt - p, cnt - p + q - pp))
            else:
                M = D // 2
                cnt = (M + 1) * (q - p) + M * (qq - pp)
                ans = max(ans, min(cnt - pp, cnt - pp + qq - p))

print(ans)