package main

import "fmt"

const inf = 1 << 60

func main() {
	var N, D int
	fmt.Scan(&N, &D)
	T := make([]int, N+1)
	K := make([]int, N+1)
	for i := 0; i < N; i++ {
		fmt.Scan(&T[i], &K[i])
	}
	dp := make([][]int, N+1)
	for i := 0; i < N+1; i++ {
		dp[i] = make([]int, 2)
		for j := 0; j < 2; j++ {
			dp[i][j] = -inf
		}
	}
	dp[0][0] = 0
	dp[0][1] = -D
	for i := 0; i < N; i++ {
		dp[i+1][0] = max(dp[i+1][0], dp[i][0]+T[i])
		dp[i+1][1] = max(dp[i+1][1], dp[i][0]+K[i]-D)
		dp[i+1][1] = max(dp[i+1][1], dp[i][1]+K[i])
		dp[i+1][0] = max(dp[i+1][0], dp[i][1]+T[i]-D)
	}
	fmt.Println(max(dp[N][0], dp[N][1]))

}

func max(a, b int) int {
	if a > b {
		return a
	}
	return b
}