#include <fstream>
#define formula rez+sobolan-(t[j]-t[shtart])*c

using namespace std;

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

int n, c, i, j, sobolan, max1;
int t[2001], p[2001], rez, shtart;

int main()
{
    f >> n >> c;
    for (i = 1; i <= n; i++)
        f >> t[i] >> p[i];

    for (i = 1; i <= n; i++)
    {
        rez = 0, shtart = 1;
        for (j = 1; j <= n; j++)
        {
            sobolan = (p[i] <= p[j] ? p[j] : 0);
            if (formula < sobolan-c)
                rez = sobolan-c, shtart = j;
            else
                rez = formula;
            if (rez > max1)
                max1 = rez;

        }
    }
    g << max1;
    return 0;
}