func main()
	var n: int :: cui@inputInt()
	var d: int :: cui@inputInt()
	var t: []int :: #[n]int
	var k: []int :: #[n]int
	for i(0, n - 1)
		do t[i] :: cui@inputInt()
		do k[i] :: cui@inputInt()
	end for
	
	var dp: [][]int :: #[n, 2]int
	do dp[0][0] :: t[0]
	do dp[0][1] :: k[0] - d
	
	for i(1, n - 1)
		do dp[i][0] :: [dp[i - 1][0], dp[i - 1][1] - d].max() + t[i]
		do dp[i][1] :: [dp[i - 1][0] - d, dp[i - 1][1]].max() + k[i]
	end for
	
	var ans: int :: dp[n - 1].max()
	do cui@print("\{ans}\n")
end func