n,m = map(int,raw_input().split())

tokyo = []
kyoto = []

for i in range(n):
	t,k = map(int,raw_input().split())
	tokyo.append(t)
	kyoto.append(k)

dp = [[0,0] for i in range(n)]

dp[0][0] = tokyo[0]
dp[0][1] = kyoto[0] - m

for i in range(1,n):
	dp[i][0] = max(dp[i-1][0] + tokyo[i],dp[i-1][1] + tokyo[i] - m)
	dp[i][1] = max(dp[i-1][1] + kyoto[i],dp[i-1][0] + kyoto[i] - m)

print max(dp[n-1])