def getlist():
	return list(map(int, input().split()))

N, M = getlist()

L = []
for i in range(N):
	ans = 0
	A, B = getlist()
	if 0 <= A and 0 <= B:
		if A < B:
			ans += (M - 1) * A + B
		else:
			ans += M * A
	elif A < 0 and 0 <= B:
		ans += B
	elif A < 0 and B < 0:
		if A * M < B:
			ans += B
		else:
			ans += A * M
	L.append(ans)

ANS = -float("inf")
val = 0
for i in range(N):
	val += L[i]
	if val > ANS:
		ANS = val

print(ANS)