#include <iostream>
#include <fstream>
#include <algorithm>
using namespace std;
ifstream f("carnati.in");
ofstream g("carnati.out");
int pret[2003],n,c,s,price,profitmax,prc,timp[1503];
struct oameni{
    int h,p;
}v[2003];
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;
        pret[i]=v[i].p;
        timp[v[i].h]=v[i].p;
    }
    sort(v+1,v+n+1,comp);
    for(int j=1;j<=n;++j)
    {
        s=0;
        price=pret[j]; ///pretul trebuie ales doar din optiunile cumparatorului
        for(int i=0;i<=1500;++i) ///subsecventa de suma maxima
        {
            if(timp[i]>=price) prc=price-c;
            else prc=-c;
            if(s<0) s=prc;
            else s+=prc;
            profitmax=max(s,profitmax);
        }
    }
    g<<profitmax;
    return 0;
}