#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;
}