#include <iostream>
#include <stdio.h>
#include <algorithm>
#include <math.h>
using namespace std;

int main(void)
{
    int n, d, tmp;
    cin >> n >> d;

    int *t = new int[n];
    int *k = new int[n];
    for (int i = 0; i < n; i++)
        cin >> t[i] >> k[i];

    int tsum = t[0];
    int ksum = k[0] - d;
    
    for (int i = 1; i < n; i++)
    {
        tmp = t[i] + max(tsum, ksum - d);
        ksum = k[i] + max(ksum, tsum - d);
        tsum = tmp;
    }

    cout << max(tsum, ksum) << endl;

    return 0;
}