## https://yukicoder.me/problems/no/818

MIN_VALUE = -10**17

def main():
    N, M = map(int, input().split())
    ab = []
    for _ in range(N):
        a, b = map(int, input().split())
        ab.append((a, b))

    if M == 1:
        array = [0] * N
        cum_ = 0
        for i in range(N):
            a,b = ab[i]
            cum_ += max(a, b)
            array[i] = cum_
        
        max_value = MIN_VALUE
        for i in range(N):
            max_value = max(max_value, array[i])
        print(max_value)
        return


    dp = [[MIN_VALUE for _ in range(8)] for _ in range(N + 1)]
    dp[0][0] = 0
    for i in range(N):
        a, b = ab[i]
        for state in range(8):
            for new_state in range(8):
                if new_state & state == state:
                    x1 = 0
                    if state == 0:
                        x1 = max(a * M, a * (M - 1) + b, b)
                    elif state == 1:
                        x1 = max(a * (M - 1), a * (M - 2) + b)
                    elif state == 2:
                        x1 = max(a * 2, a + b, b)
                    elif state == 3:
                        x1 = max(a , b)
                    elif state == 4:
                        x1 = max(a * (M - 1), b)
                    elif state == 5:
                        x1 = a * (M - 2)
                    elif state == 6:
                        x1 = max(a, b)


                    dp[i +1][new_state] = max(dp[i + 1][new_state], dp[i][state] + x1)
    print(max(dp[-1]))

            






if __name__ == "__main__":
    main()