import sys
input = sys.stdin.readline

N, D = map(int, input().split())
P, Q, R = [0] * N, [0] * N, [0] * N
for i in range(N):
    P[i], Q[i] = map(int, input().split())
    R[i] = Q[i] - P[i]

ans = -10**18
for i in range(N):
    for j in range(N):
        if i == j:
            continue
        if R[i] >= 0 and R[j] >= 0:
            mi = min(-P[i], R[i] - P[j])
        else:
            now = (R[i] + R[j]) * ((D-1)//2)
            if D % 2:
                mi = -P[i]
            else:
                mi = min(-P[i], R[i] - P[j])
            if now <= 0:
                mi += now
        ans = max(ans, mi)

print(ans)