from itertools import accumulate
N, M = map(int, input().split())
AB = [tuple(map(int, input().split())) for _ in range(N)]
K1 = [max(t) for t in AB]
K3 = list(accumulate(K1))
ks = sum(K1)
K1 = [0] + list(accumulate(K1[::-1]))
K1 = list(accumulate(K1, min))[::-1][1:]
K2 = [max(a*M, a*(M-1)+b, b) for (a, b) in AB]
K2 = list(accumulate(K2))
print(max(j-i+ks-k for i, j, k in zip(K1, K2, K3)))