import numpy as np

A = []
B = []

N, M = map(int, input().strip().split(' '))

for i in range(N):
    a, b = map(int, input().strip().split(' '))
    A.append(a)
    B.append(b)

A = np.array(A)
B = np.array(B)

S0 = np.maximum(np.maximum(A * M, A * (M - 1) + B), B)
S1 = np.maximum(A, B)

ans = None

for k in range(1, N+1):
    score = S0[:k].sum()
    if k != N:
        score += np.cumsum(S1[k:]).max()

    if ans is None:
        ans = score

    if score > ans:
        ans = score

print(ans)