#include<cstdio>
#include<utility>
#include<algorithm>
using namespace std;

int n,c,i,cump,pret,idx,sum,SOL;

pair<int,int> V[2010];

void read(),solve();

int main()
{
	read();
	solve();
	
	return 0;
}

void read()
{
	freopen("carnati.in","r",stdin);
	freopen("carnati.out","w",stdout);
	scanf("%d%d",&n,&c);
	for(i=1;i<=n;i++)
	{
		scanf("%d%d",&V[i].first,&V[i].second);
	}
}

void solve()
{
	sort(V+1,V+n+1);
	idx=V[1].first;
	pret=V[1].second;
	SOL=pret-c;
	cump=1;
	for(i=2;i<=n;i++)
	{
		if(V[i].second>=pret)
		{
			cump++;
			sum=cump*pret-(V[i].first-idx+1)*c;
			if( sum>V[i].second-c)
			{
				if(SOL<sum)SOL=sum;
				else ;
			}
			else
			{
				cump=1;
				idx=V[i].first;
				pret=V[i].second;
				sum=pret-c;
				if(SOL<sum)SOL=sum;
			}
		}
		else
		{
			if( sum > (cump+1)*V[i].second-(V[i].first-idx+1)*c)continue;
			else
			{
				pret=V[i].second;
				cump++;
				sum=cump*pret-(V[i].first-idx+1)*c;
				if(SOL<sum)SOL=sum;
			}
		}
	}
	printf("%d\n",SOL);
}