#include <fstream>
#include <cstring>
using namespace std;
ifstream cin("carnati.in");
ofstream cout("carnati.out");

int n,i,a,b,j,pivot,pret,start,aux,plata,finish,profit,ora,Pbest(-99999999),cumparatori(1),T[10000],P[10000];
int main()
{
    cin>>n>>plata;
    for (i=1;i<=n;++i) cin>>T[i]>>P[i];
    Pbest=P[1]-plata;
    start=T[1];
    finish=T[1];
    pret=P[1];
    for (i=2;i<=n;++i){
                       cin>>T[i]>>P[i];
                       if (P[i]>pret) ++cumparatori;
                       if (cumparatori*pret-(T[i]-start+1)*plata > Pbest ) {
                                                                          Pbest=cumparatori*pret-(T[i]-start+1)*plata ;
                                                                          }
                       if (P[i]-plata > Pbest) {
                                                  Pbest=P[i]-plata;
                                                  start=T[i];
                                                  pret=P[i];
                                                  cumparatori=1; 
                                                  }
                       aux=1;
                       for (j=i-1;j>=1;--j) {
                                             if (P[j]>=P[i]) ++aux;
                                             if (aux*P[i]-(T[i]-T[j]+1)*plata > Pbest ) {
                                                                                         Pbest=aux*P[i]-(T[i]-T[j]+1)*plata ;
                                                                                         pret=P[i];
                                                                                         cumparatori=aux;
                                                                                         start=T[j];
                                                                                         }
                                             }
                      }
    cout<<Pbest<<"\n";
return 0;
}