#include<stdio.h>
FILE*fin=fopen("carnati.in","r");
FILE*fout=fopen("carnati.out","w");
int n,p[2001],t[2001],c;
void rech(int nod,int dim)
{
  int nd=nod,max=t[nod];
  int st=nod<<1,dr=st+1;
  if(t[st]>max&&st<=dim){max=t[st];nd=st;}
  if(t[dr]>max&&dr<=dim){max=t[dr];nd=dr;}
  if(nd!=nod)
  {
    t[nd]^=t[nod]^=t[nd]^=t[nod];
    p[nd]^=p[nod]^=p[nd]^=p[nod];
    rech(nd,dim);
  }
}
void ord()
{
  int i;
  for(i=n/2;i>=1;i--)
    rech(i,n);
  int dim=n;
  while(dim>1)
  {
    t[1]^=t[dim]^=t[1]^=t[dim];
    p[1]^=p[dim]^=p[1]^=p[dim];
    dim--;
    rech(1,dim);
  }
}
int main()
{
  int i,j,pf;
  long long maxl,sp,maxr,best;
  fscanf(fin,"%d%d",&n,&c);
  for(i=1;i<=n;i++)
    fscanf(fin,"%d%d",&t[i],&p[i]);
  fclose(fin);
  ord();
  best=0;
  for(i=1;i<=n;i++)
  {
    pf=p[i];
    sp=0;
    maxr=0;
    for(j=i;j>=1;j--)
    {
      if(p[j]>=pf) sp+=pf;
      if(sp-(t[i]-t[j]+1)*c>maxr) maxr=sp-(t[i]-t[j]+1)*c;
    }
    sp=0;
    maxl=0;
    for(j=i+1;j<=n;j++)
    {
      if(p[j]>=pf) sp+=pf;
      if(sp-(t[j]-t[i])*c>maxl) maxl=sp-(t[j]-t[i])*c;
    }
    if(maxl+maxr>best) best=maxl+maxr;
  }
  fprintf(fout,"%lld",best);
  fclose(fout);
  return 0;
}