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#include <algorithm>
#include <iostream>
#include <unordered_map>
#include <vector>

using namespace std;

struct Styl {
  vector<int> a_list;
  vector<long long> prefix_max;

  void add(int a, long long wynik) {
    a_list.push_back(a);
    if (prefix_max.empty()) {
      prefix_max.push_back(wynik);
    } else {
      prefix_max.push_back(max(prefix_max.back(), wynik));
    }
  }

  long long query(int threshold) {
    int lewo = 0, prawo = a_list.size() - 1;
    int pos = a_list.size();
    while (lewo <= prawo) {
      int mid = (lewo + prawo) / 2;
      if (a_list[mid] <= threshold) {
        pos = mid;
        prawo = mid - 1;
      } else {
        lewo = mid + 1;
      }
    }
    if (pos == 0)
      return 0;
    return prefix_max[pos - 1];
  }
};

int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);

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

  vector<pair<int, int>> klocki(n);
  for (int i = 0; i < n; ++i) {
    cin >> klocki[i].first >> klocki[i].second;
  }

  unordered_map<int, Styl> styl_mapa;
  Styl global_list;
  long long max_wynik = 0;

  for (int i = n - 1; i >= 0; --i) {
    int a = klocki[i].first;
    int w = klocki[i].second;

    Styl &styl_lista = styl_mapa[w];
    long long best_same = styl_lista.query(a);
    long long naj_glob = global_list.query(a);
    long long naj_inne = naj_glob > 0 ? naj_glob - c : 0;

    long long current = a + max(best_same, naj_inne);
    max_wynik = max(max_wynik, current);

    styl_lista.add(a, current);
    global_list.add(a, current);
  }

  cout << max_wynik << "\n";
//:p
  return 0;
}