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, dtype=np.int64)
B = np.array(B, dtype=np.int64)

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

S = np.cumsum(S0)

for k in range(1, N+1):
    if k > N-2:
        continue
    S[k] += np.cumsum(S1[k:])[1:].max()

print(S[1:].max())