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

int n,c,i,j,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);
	for(i=1;i<=n;i++)
	{
		pret=V[i].second;
		sum=-1;
		for(j=1;j<=n;j++)
		{
			if(V[j].second<pret)continue;
			if(sum<0)
			{
				sum=pret-c;
				idx=V[j].first;
				cump=1;
			}
			else
			{
				cump++;
				sum=cump*pret-(V[j].first-idx+1)*c;
				if(SOL<sum)SOL=sum;
			}
		}
	}
	printf("%d\n",SOL);
}