#include<iostream>
#include<fstream>
#include<algorithm>
using namespace std;
int a;
struct vector {
	int t,p;
};
inline bool cmp (const vector a, const vector b)
{
	return a.t<b.t;
}
vector v[2001];
int ssm(int pret, int n)
{
	int j,max,s;
	max=0;
	s=0;
	for(j=1;j<=n;j++) {
		if(j>1)
			s=s-(v[j].t-v[j-1].t-1)*a;
		if(s<0) 
			s=0;
		if(v[j].p>=pret)
			s=s+pret;
		s=s-a;
		if(s>max)
			max=s;
	}
	return max;
}
int main ()
{
	int n,i,max,s;
	ifstream f("carnati.in");
	ofstream g("carnati.out");
	f>>n>>a;
	for(i=1;i<=n;i++)
		f>>v[i].t>>v[i].p;
	f.close();
	sort(v+1,v+n+1,cmp);
	max=0;
	for(i=1;i<=n;i++) {
		s=ssm(v[i].p,n);
		if(s>max)
			max=s;
	}
	g<<max;
	g.close();
	return 0;
}