#include<iostream>
#include<fstream>
#include<algorithm>
using namespace std;
long a;
struct vector {
	long t,p;
};
inline bool cmp (const vector a, const vector b)
{
	if(a.t==b.t)
		return a.p>b.p;
	return a.t<=b.t;
}
vector v[2001];
long ssm(long pret, long n)
{
	long j,i;
	long max,s,c,b;
	max=0;
	s=0;
	c=v[1].t;
	b=1;
	for(j=1;j<=n;j++) {
			s=s-(v[j].t-c+1)*a;
			if(v[j].p>=pret)
				s=s+pret;
			if(s<0) {
				c=v[j+1].t;
				s=0;
				b=0;
			}
			if(s>max)
				max=s;
			if(b==1)
				s=s+(v[j].t-c+1)*a;
			else b=1;
	}
	return max;
}
int main ()
{
	long n,i;
	long 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;
}