#include<fstream>
#include<algorithm>
using namespace std;
ifstream f("carnati.in");
ofstream g("carnati.out");
int i,n,cost,x,u,s,c,j,sol=-100000;
struct cl
{
	int t,p;
};
cl a[2001];
int main()
{
    f>>n>>c;
    for(i=1;i<=n;++i)
    f>>a[i].t>>a[i].p;
    sort(a+1,a+n+1);
    a[0].t=a[0].p=-1000;
    for(i=1;i<=n;++i)
    {
        s=0;
        for(j=1;j<=n;++j)
        {
            cost=a[i].p;
            x=cost-(a[j].t-a[j-1].t)*c;
            if(s+x<cost-c)
            s=cost-c;
            else
            s=s+x;
            sol=max(sol,s);
        }
 
    }
    g<<sol<<"\n";
    return 0;
}