import sys
if sys.platform =='ios':
	import clipboard
	a=clipboard.get()
	a = a.split('\n')
	text = '\n'.join(a)
	with open('input_file.txt','w') as f:
		f.write(text)
	sys.stdin = open('input_file.txt')
sys.setrecursionlimit(410000000)
stdin = sys.stdin 
ni = lambda: int(ns())
ni_1 = lambda: int(ns())-1
na = lambda: list(map(int, stdin.readline().split()))
ns = lambda: stdin.readline().strip()
nm = lambda: map(int,input().split())
na_1 = lambda: list(map(lambda x:int(x)*(-1), stdin.readline().split()))
na_2 = lambda: list(map(lambda x:int(x)-1, stdin.readline().split()))
N,D = nm()
dp = [[0,0] for i in range(N+1)]
dp[0][1] = -10**10
for i in range(1,N+1):
	t,k = nm()
	dp[i][0] = max(dp[i-1][0]+t,dp[i-1][1]-D+t)
	dp[i][1] = max(dp[i-1][1]+k,dp[i-1][0]-D+k)
		
		
print(max(dp[-1]))