#include <fstream>
#include <algorithm>

using namespace std;

ifstream f("carnati.in");
ofstream g("carnati.out");

int n, c, i, j, s, maxim, p, max1;
struct carne
{
    int pret, timp;
}a[2001];

bool cmp(carne a, carne b)
{
    return (a.timp < b.timp);
}

int main()
{
    f >> n >> c;
    for (i = 1; i <= n; i++)
        f >> a[i].timp >> a[i].pret;

    sort(a+1, a+n+1, cmp);

    for (i = 1; i <= n; i++)
    {
        max1 = a[i].pret, s = 0;
        for (j = 1, s = 0; j <= n; j++)
            {
                p = 0;
                if (a[j].pret >= max1)
                    p = max1;
                if (s - (a[j].timp - a[j-1].timp)*c > 0)
                    s += p - (a[j].timp - a[j-1].timp)*c;
                else
                    s = p-c;
                if (s > maxim)
                    maxim = s;
            }
    }
    g << maxim;
    return 0;
}