#include <stdio.h>

long max, profit, i, n, maxim, max2, j, pret, c, v[2048], m[2048];

struct lol
{
	long h, p;
};
lol buyer[2048];

int main()
{
	freopen ("carnati.in", "rt", stdin);
	freopen ("carnati.out", "wt", stdout);

	scanf("%ld %ld", &n, &c);
	for (i = 1; i <= n; i ++)
	{
		scanf("%ld %ld", &buyer[i].h, &buyer[i].p);
		v[buyer[i].h] = i;
		max = buyer[i].h > max ? buyer[i].h : max;
		max2 = buyer[i].p > max2 ? buyer[i].p : max2;
	}

	for (j = 1;	j <= n; j ++)
	{
		pret = buyer[j].p;
		profit = 0;
		for (i = 1; i <= max; i ++)
		{
			if (pret <= buyer[v[i]].p)
				profit += pret - c;
			else
				profit -= c;
			if (profit < 0)
				profit = 0;
			maxim = profit > maxim ? profit : maxim;
		}
	}

	printf("%ld\n", maxim);

	return 0;
}