#include<fstream>

using namespace std;

ifstream fin("carnati.in");
ofstream fout("carnati.out");

long n,c,g,d,maxim,h[4002],s;
struct client { long t,p;
              } a[2001];

int main()
{   long i,j;
     fin>>n>>c;
     for(i=1;i<=n;i++)
         fin>>a[i].t>>a[i].p;
     for(i=1;i<=n;i++)
     {   s=0;
         for(j=1;j<=n;j++)
         {   if(a[j].p>=a[i].p) g=a[i].p;
               else g=0;
              d=s-(a[j].t-a[j-1].t)*c+g;
              if(d<g-c) d=g-c;
              if(maxim<d) maxim=d;
              s=d;
         }
     }
     fout<<maxim;
     fin.close();
     fout.close();
     return 0;
}