n, d = map(int, input().split())
p = [0] * n
q = [0] * n
for i in range(n):
    p[i], q[i] = map(int, input().split())
if d == 1:
    ans = -1000000000
    for i in range(n):
        if ans < -p[i]:
            ans = -p[i]
    print(ans)
    exit(0)
ans = -1000000000
for i in range(n):
    for j in range(n):
        if i == j:
            continue
        res = -p[i]
        if d % 2 == 0:
            res = min(res, (q[i] - p[i] + q[j] - p[j]) * (d // 2) - q[j])
        else:
            res = min(res, (q[i] - p[i] + q[j] - p[j]) * (d // 2) - p[j])
        if ans < res:
            ans = res
print(min(ans, 0))