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#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef long double ld;
typedef vector<int> vi;
typedef pair<ll, ll> pii;

#define FOR(x, b, e) for(int x = (b); x <= (e); ++(x))
#define FORD(x, b, e) for(int x = (b); x >= (e); --(x))
#define REP(x, n) for(int x = 0; x < (n); ++x)
#define ALL(c) (c).begin(), (c).end()
#define SIZE(x) ((int)(x).size())
#define FOREACH(a, b) for (auto&a : (b))
#define PB push_back
#define PF push_front
#define ST first
#define ND second
#define DBG(vari) cerr<<#vari<<" = "<<(vari)<<endl

template <typename T>
std::ostream& operator<<(std::ostream &output, const vector<T> &vec)
{
    output << "[";
    FOREACH(x, vec) output << x << ", ";
    output << "]";
    return output;
}

template <typename T, typename U>
std::ostream& operator<<(std::ostream &output, const pair<T,U> &p)
{
    output << "(";
    output << p.ST << ", " << p.ND;
    output << ")";
    return output;
}


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

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

  vector<pii> aw(n);
  REP(i, n) cin >> aw[i].ST >> aw[i].ND;
  sort(ALL(aw));
  reverse(ALL(aw));

  ll najl_mn=0;
  vector<ll> najl_w(500001, 0);

  ll cand_mn=0;
  vector<pii> cand_najw;

  REP(i, SIZE(aw)) {
    ll wyn = max(najl_mn + aw[i].ST - c, najl_w[aw[i].ND] + aw[i].ST);

    cand_mn = max(cand_mn, wyn);
    cand_najw.emplace_back(aw[i].ND, wyn);
         
    if (i + 1 == SIZE(aw) || aw[i+1].ST < aw[i].ST) {

      najl_mn = max(najl_mn, cand_mn); 

      REP(j, SIZE(cand_najw)) {
        ll w, ww; tie(w, ww) = cand_najw[j];
        najl_w[w] = max(najl_w[w], ww);
      }

      cand_najw.clear();
    }
  }

  cout << najl_mn << endl;

}