#include <bits/stdc++.h>

using namespace std;
ifstream fin("carnati.in");
ofstream fout("carnati.out");
struct Vrajeala
{
    int timp, avere;
}a[2005];
bool SortVrajealaByTime(Vrajeala o, Vrajeala p)
{
    return o.timp < p.timp;
}
int n, c, sol = INT_MIN, sum;
int fr[1505];
int main()
{
    int i, j, pret, x;
    fin >> n >> c;
    for (i = 1; i <= n; i++)
        fin >> a[i].timp >> a[i].avere;
    sort(a + 1, a + n + 1, SortVrajealaByTime);
    a[0].timp = a[1].timp;
    for (i = 1; i <= n; i++)
    {
        pret = a[i].avere;
        sum = 0;
        for (j = 1; j <= n; j++)
        {
            x = sum - (a[j].timp - a[j - 1].timp) * c;
            if (x < 0)
                sum = 0;
            else sum = x;
            if (a[j].avere >= pret)
                sum += pret;
            sol = max(sol, sum);
        }
    }
    fout << sol - c;

    return 0;
}