def larger(xl, yl):
	if xl[1] > yl[1]: return xl
	else: return yl

N, D = map(int, raw_input().split())
salarylist = []
for i in range(0, N):
	salarylist.append(map(int, raw_input().split()))
dp = [[0, 0]]
for i in range(0, N):
	if dp[i][0] == 0:
		dp.append(larger([0, dp[i][1]+salarylist[i][0]], [1, dp[i][1]+salarylist[i][1]-D]))
	else: dp.append(larger([0, dp[i][1]+salarylist[i][0]-D], [1, dp[i][1]+salarylist[i][1]]))

print dp[N][1]