import math

N, D = map( int, raw_input().split( ) )
okane = []
incomes = []
for i in range(N):
	incomes.append( list(map( int, raw_input().split( ))) )
pos = 0

okane.append([0,-D])
for i in range(N):
	tk = max( incomes[i][0]+okane[i][0] , incomes[i][0]+okane[i][1] - D )
	kk = max( incomes[i][1] + okane[i][1], incomes[i][1] + okane[i][0] - D )
	inc = [tk, kk]
	okane.append(inc)

al = max(okane[N][0], okane[N][1])
print al