#include <stdio.h>
#include <algorithm>
using namespace std;
struct customer{int t, p;};
customer v[2010];
int t[2010], p[2010], a[2010];
bool cmp (customer a, customer b) {
  return a.t < b.t;
}
int main()
{
  FILE *fin, *fout;
  int n, c, i, j, mx;
  fin = fopen ("carnati.in", "r");
  fout = fopen ("carnati.out", "w");
  fscanf (fin, "%d%d", &n, &c);
  for (i = 1; i <= n; i++)
    fscanf (fin, "%d%d", &v[i].t, &v[i].p);
  sort (v + 1, v + n + 1, cmp);
  mx = 0;
  for ( i = 1; i <= n; i++) {
    for (j = 1; j <= n; j++) {
      if ( v[i].p <=  v[j].p )
        a[j] = max (a[j - 1] - (v[j].t - v[j - 1].t) * c + v[i].p, v[i].p - c);
      else
        a[j] = max (0, a[j - 1] - (v[j].t - v[j - 1].t) * c);
      if (mx < a[j])
        mx = a[j];
    }
  }
  fprintf (fout, "%d\n", mx);
  fclose (fin);
  fclose (fout);
  return 0;
}