#include<fstream>
#include<algorithm>

using namespace std;

ifstream in("carnati.in");
ofstream out("carnati.out");

const int N = 1<<11;

struct client
{
	short int t;
	int p;
};

client v[N];
int n,c;

inline int max(int x,int y)
{
	return x>y ? x : y;
}

void citire()
{
	in>>n>>c;
	for(short int i=1 ; i<=n ; ++i)
		in>>v[i].t>>v[i].p;
	in.close();
}

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

int profit(int pret)
{
	int i,sc,smax;
	if(v[1].p>=pret)
		sc=smax=pret-c;
	else
		sc=smax=-c;
	for(i=2 ; i<=n ; ++i)
	{
		if(sc < (v[i].t-v[i-1].t-1)*c)
			sc=0;
		else
			sc-=(v[i].t-v[i-1].t-1)*c;
		if(v[i].p>=pret)
			sc+=pret;
		sc-=c;
		smax = max(smax,sc);
	}
	return smax;
}

int calcul()
{
	int maxx=0;
	short int i;
	for(i=1 ; i<=n ; ++i)
		maxx=max(maxx,profit(v[i].p));
	return maxx;
}

int main()
{
	citire();
	sort(v+1,v+n+1,cmp);
	out<<calcul()<<"\n";
	out.close();
	return 0;
}