N, M = map(int, input().split())
A = []
B = []
for i in range(N):
    a, b = map(int, input().split())
    A.append(a)
    B.append(b)
sum = [0] * N
for i in range(N):
    sum[i] = sum[i - 1] + max(A[i], B[i])
ans = -10 ** 18
cur = 0
m = -10 ** 18
for i in range(N):
    cur += max([A[i] * M, A[i] * (M - 1) + B[i], B[i]])
    m = max(m, cur - sum[i])
    ans = max(ans, m + sum[i])
print(ans)