package yukicoder;

import java.util.Scanner;

public class Main {
	
    private static int[] tokyo;
	private static int[] kyoto;
	private static int days;
	private static int exp;
	private static int[][] dp;

	public static void main(String[] args) {
        // 自分の得意な言語で
        // Let's チャレンジ！！
        Scanner sc = new Scanner(System.in);
        days =  sc.nextInt();
        exp = sc.nextInt();
        tokyo = new int[days];
        kyoto = new int[days];
        dp = new int[days][2];
        for(int i = 0 ; i < days ; i++){
        	tokyo[i] = sc.nextInt();
        	kyoto[i] = sc.nextInt();
        }
        int gain = greed();
        System.out.println(gain);
    }

	

	private static int greed() {
		dp[0][0] = tokyo[0];
		dp[0][1] = kyoto[0] - exp;
        for(int i = 0 ; i < days-1 ; i++){
        	dp[i+1][0] = max(dp[i][0] , dp[i][1] - exp) + tokyo[i+1];
        	dp[i+1][1] = max(dp[i][0] - exp , dp[i][1]) + kyoto[i+1];
        }
        int tokyo = dp[days-1][0];
        int kyoto = dp[days-1][1];
		return tokyo > kyoto ? tokyo : kyoto;
	}



	private static int max(int i, int j) {
		return i > j ? i : j;
	}
}