#include <iostream>
#include <fstream>
#include <algorithm>
using namespace std;
ifstream f("carnati.in");
ofstream g("carnati.out");
long long n,c,s,price,profitmax,prc,cost,r;
struct oameni{
    int h,p;
}v[2005];
bool comp(oameni a, oameni b)
{
    return (a.h<b.h);
}
int main()
{
    f>>n>>c;
    for(int i=1;i<=n;++i)
    {
        f>>v[i].h>>v[i].p;
    }
    sort(v+1,v+n+1,comp);
    v[0].h=-10;
    for(int j=1;j<=n;++j)
    {
        s=0;
        price=v[j].p;
        profitmax=max(profitmax,s);
        for(int i=1;i<=n;++i)
        {
            if(v[i].p>=price) prc=price-c;
            else prc=-c;

            cost=v[i].h-v[i-1].h-1;

            r=max(s+prc-cost*c, prc);
            s=r;
            profitmax=max(profitmax,r);
        }
    }
    g<<profitmax;
    return 0;
}