#include <iostream>
#include <fstream>
using namespace std;
ifstream f("carnati.in");
ofstream g("carnati.out");
int n,c,t[2005],p[2005];
void Read()
{ int i;
   f>>n>>c; //cout<<n<<" "<<c;
  for(i=1;i<=n;i++)
    f>>t[i]>>p[i]; //cout<<p[1];
}
int Ans(int price)
{ int i,mn,set=0,nr=0,sol=0;
   for(i=1;i<=n;i++)
   { if (price<=p[i])
      {nr++; //cout<<p[i]<<"\n";
      if (set)
       {if (nr*price-t[i]*c-mn>sol) sol=nr*price-t[i]*c-mn;
         mn=min(mn,nr*price-t[i]*c+c-price);
       }
      else {set=1;mn=nr*price-t[i]*c+c-price;}
        if (nr*price-t[i]*c>sol) sol=nr*price-t[i]*c;
      }
     //if (i==2) cout<<mn;
  }
return sol;
}
int main()
{ int i,res=-(1<<31);
    Read();
     for(i=1;i<=n;i++)
      res=max(res,Ans(p[i]));
    g<<res;
    return 0;
}