# input
n, d = map(int, input().split())
price = [ list(map(int, input().split())) for i in range(n) ]

# dynamic programming
dp = [ 0, -(10 ** 20) ]
for t, k in price:
	dp = [ max(dp[0], dp[1] - d) + t, max(dp[0] - d, dp[1]) + k ]

# output
print(max(dp))