#include<fstream>
#include<algorithm>
using namespace std;
ifstream f("carnati.in");
ofstream g("carnati.out");
int n,m,c,i,j;
long long smax,yy,y,k;

struct qual{
int t,x;
}a[2002],aux;

bool cmp(const qual &x,const qual &y)
{
    return x.t<y.t;
}

int main()
{
 f>>n>>c;
 for(i=1;i<=n;i++)
 f>>a[i].t>>a[i].x;

 sort(a+1,a+n+1,cmp);
 a[0].x=a[0].t=-10;
 for(i=1;i<=n;i++)
 {
     k=0;
     for(j=1;j<=n;j++)
     {
         if(a[j].x>=a[i].x) y=a[i].x;
         else y=0;
         yy=k-(a[j].t-a[j-1].t)*c+y;
         if(yy<y-c) yy=y-c;
         if(yy>smax) smax=yy;
         k=yy;
     }
 }

 g<<smax;

 return 0;
}