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)

ans = 0

for k in range(N):
    score = 0
    for i in range(k):
        score += max(A[i] * M,
                     A[i] * (M - 1) + B[i],
                     B[i])
    max_score = score
    for i in range(k, N):
        score += max(A[i], B[i])
        if score > max_score:
            max_score = score
    if max_score > ans:
        ans = max_score

print(ans)