#include <bits/stdc++.h>
using namespace std;
using ll = long long int;
using ull = unsigned long long int;
using ld = long double;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
int dx[] = {1, 0, -1, 0, 1, -1, 1, -1};
int dy[] = {0, 1, 0, -1, 1, 1, -1, -1};
const int INT_INF = (int)(2e9);
const ll LL_INF = (ll)(2e18);
static mt19937 _g(time(nullptr));
inline ll randint(ll a, ll b) {
  ll w = (_g() << 31LL) ^ _g();
  return a + w % (b - a + 1);
}
inline void fast_io() {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr);
  cout.tie(nullptr);
};
template <typename T, typename S>
inline ostream& operator<<(ostream& os, const pair<T, S> p) {
  cout << "[" << p.first << ";" << p.second << "]";
  return os;
}
template <typename T, typename S>
inline ostream& operator<<(ostream& os, const map<T, S> p) {
  for (auto el : p) cout << "[" << el.first << ";" << el.second << "]";
  return os;
}
template <typename T>
inline ostream& operator<<(ostream& os, const vector<T>& v) {
  for (auto el : v) cout << el << " ";
  return os;
}
template <typename T>
inline vector<T> fetch_vec(int sz) {
  vector<T> ret(sz);
  for (auto& elem : ret) cin >> elem;
  return ret;
}
int N;
ll C;
vector<ll> X;
vector<ll> V, sumV, revsumV;
void input() {
  fast_io();
  cin >> N >> C;
  for (int i = 0; i < N; i++) {
    ll x, v;
    cin >> x >> v;
    X.push_back(x);
    V.push_back(v);
  }
}
int solve() {
  sumV.assign(N + 1, 0);
  revsumV.assign(N + 1, 0);
  for (int i = 0; i < N; i++) sumV[i + 1] = sumV[i] + V[i];
  for (int i = N - 1; i >= 0; i--) revsumV[i] = revsumV[i + 1] + V[i];
  ll ans = 0;
  for (int i = 0; i < N; i++) {
    ans = max(ans, (sumV[i + 1] - X[i]));
  }
  for (int i = N - 1; i >= 0; i--) {
    ans = max(ans, revsumV[i + 1] - (C - X[i]));
  }
  cout << ans << endl;
  return 0;
}
int main() {
  input();
  solve();
  return 0;
}
