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#include <bits/stdc++.h>
using namespace std;

#define ll long long
constexpr int maxn = 5e5+2;

ll res,act, best[maxn], best_overall;
vector <int> ls[maxn];

int main()
{
	ios_base::sync_with_stdio(0);
	cin.tie(0); cout.tie(0);

  int n,c; cin >> n >> c;

	for(int i=1; i<=n; i++)
  {
    int a,w; cin >> a >> w;
    ls[a].push_back(w);
  }

  vector <pair<ll,int>> tmp;
  for(int i=1; i<=5e5; i++)
  {
    if(!(int)ls[i].size()) continue;
    tmp.clear();

    for(auto w : ls[i])
    {
      act = max(best[w]+i,best_overall+i-c);
      tmp.push_back({act,w});
      res = max(res, act);
    }
    for(auto el : tmp)
    {
      best_overall = max(best_overall, el.first);
      best[el.second] = max(best[el.second],el.first);
    }
  }
  cout << res << '\n';
}