#include<fstream>
using namespace std;
ifstream f("carnati.in");
ofstream g("carnati.out");
int n,c,costmax,cost,i;
struct carnati
{
    int t,p;
};
carnati a[2001];
int pret(int cost1)
{
    int costmax1=-100,sc=-1,i,costc=0;
    for(i=1;i<=n;i++)
    {
        if(a[i].p<cost)
        break;
        if(sc!=-1)
        {
            costc=costc-(a[i].t-a[sc].t-1)*c;
			if(costc<0)
				costc=0;
		}
		else
			costc=0;
			sc=i;
			costc=costc+(cost-c);
			if(costmax1<costc)
				costc=costmax1;
        }
	return costmax1;
}
int main()
{
    f>>n>>c;
    for(i=1;i<=n;i++)
    f>>a[i].t>>a[i].p;
	for(i=1;i<=n;i++)
	{
		cost=pret(a[i].p);
		if(costmax<cost)
			costmax=cost;
	}
	g<<costmax<<" ";
	return 0;
}