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

using namespace std;

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

int n, c, i, j, G, max1;
int t[2001], p[2001], rez, r1, incep;

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

    for (i = 1; i <= n; i++)
    {
        rez = 0, incep = 1;
        for (j = 1; j <= n; j++)
        {
            if (p[i] <= p[j])
                G = p[j];
            else
                G = 0;
            rez += G;

            if (formula >= G-c)
                r1 = formula;
            else
                r1 = G-c, incep = 1;

            if (r1 > max1)
                max1 = r1;

        }
    }
    g << max1;
    return 0;
}