package yukicoder;

import java.util.Scanner;
import static java.lang.Math.max;

public class No561 {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        int n = sc.nextInt(), d = sc.nextInt();
        int t, k, beforeT, beforeK;
        int resultT = 0, resultK = 0;
        boolean tokyo = true;

        beforeT = sc.nextInt();
        beforeK = sc.nextInt() - d;
        resultT = beforeT;
        resultK = beforeK;

        for(int i = 0; i < n - 1; i++) {
            t = sc.nextInt();
            k = sc.nextInt();
            resultT = t + max(beforeT, beforeK - d);
            resultK = k + max(beforeK, beforeT - d);
            beforeT = resultT;
            beforeK = resultK;
        }

        System.out.println(max(resultT, resultK));
    }
}