import math

N, D = map( int, raw_input().split( ) )
okane = 0
incomes = []
for i in range(N):
	incomes.append( list(map( int, raw_input().split( ))) )
pos = 0
for i in range(N):
	if pos == 0:
		if incomes[i][0] + D < incomes[i][1]:
			okane += incomes[i][1] - D
			pos = 1
		else:
			okane += incomes[i][0]
	elif pos == 1:
		if incomes[i][1] + D < incomes[i][0]:
			okane += incomes[i][0] - D
			pos = 0
		else:
			okane += incomes[i][1]

print okane