# 嘘解法

# 適当に組み合わせて、嘘貪欲

INF = 10 ** 9

N, D = map(int, input().split())
PQs = [tuple(map(int, input().split())) for _ in range(N)]

answer = -INF
for i in range(N):
    P1, Q1 = PQs[i]
    for j in range(i + 1, N):
        P2, Q2 = PQs[j]
        delta = -P1+Q1-P2+Q2
        tmp = 0
        tmp = min(-P1, -P1+Q1-P2)
        v = delta * ((D - 1) // 2)
        D -= 1
        D %= 2
        D += 1
        for i in range(D):
            if i % 2 == 0:
                P, Q = P1, Q1
            else:
                P, Q = P2, Q2
            v -= P
            tmp = min(tmp, v)
            v += Q
        answer = max(answer, tmp)
print(answer)