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

#define fs first
#define sc second
#define pb emplace_back
#define mp make_pair
#define all(v) v.begin(), v.end()
#define rall(v) v.rbegin(), v.rend()

const int32_t inf = 1001001001;
const int64_t infll = 1001001001001001001ll;
const int dx[] = {0, -1, 1, 0, -1, 1, -1, 1}, dy[] = {-1, 0, 0, 1, -1, -1, 1, 1};
const string YES = "YES", NO = "NO", Yes = "Yes", No = "No", yes = "yes", no = "no";

// clang-format off
// ostream &operator<<(ostream &os, __int128_t value) { if (ostream::sentry(os)) { __uint128_t tmp = value < 0 ? -value : value; char buffer[64]; char *d = end(buffer); do { --d; *d = "0123456789"[tmp % 10]; tmp /= 10; } while (tmp != 0); if (value < 0) { --d; *d = '-'; } int len = end(buffer) - d; if (os.rdbuf()->sputn(d, len) != len) { os.setstate(ios_base::badbit); }} return os; }
// istream &operator>>(istream &is, __int128_t &value) { string in; is >> in; value = 0; for (const char &c : in) { if ('0' <= c && c <= '9') value = 10 * value + (c - '0'); } if (in[0] == '-') value *= -1; return is; }
// ostream &operator<<(ostream &os, __uint128_t value) { if (ostream::sentry(os)) { char buffer[64]; char *d = end(buffer); do { --d; *d = "0123456789"[value % 10]; value /= 10; } while (value != 0); int len = end(buffer) - d; if (os.rdbuf()->sputn(d, len) != len) { os.setstate(ios_base::badbit); }} return os; }
// istream &operator>>(istream &is, __uint128_t &value) { string in; is >> in; value = 0; for (const char &c : in) { if ('0' <= c && c <= '9') value = 10 * value + (c - '0'); } return is; }
template <typename T> ostream &operator<<(ostream &os, vector<T> &v) { os << v[0]; for (int i = 1; i < v.size(); ++i) os << " " << v[i]; return os; }
template <typename T> istream &operator>>(istream &is, vector<T> &v) { for (T &in : v) is >> in; return is; }
template <typename T1, typename T2> ostream &operator<<(ostream &os, pair<T1, T2> p) { os << p.fs << " " << p.sc; return os; }
template <typename T1, typename T2> istream &operator>>(istream &is, pair<T1, T2> &p) { is >> p.fs >> p.sc; return is; }

struct IoSetup { IoSetup(){ cin.tie(0); ios::sync_with_stdio(0); cout << fixed << setprecision(10); cerr << fixed << setprecision(10); } } iosetup;

inline int64_t in() { int64_t x = 0; cin >> x; return x; }

template <typename T> void sort(vector<T> &v) { sort(all(v)); }
template <typename T> vector<T> make_v(size_t a, T b) { return vector<T>(a, b); }
template <typename... Ts> auto make_v(size_t a, Ts... ts) { return vector<decltype(make_v(ts...))>(a, make_v(ts...)); }
template <typename T1, typename T2> inline bool chmax(T1 &a, T2 b) { return a < b && (a = b, true); }
template <typename T1, typename T2> inline bool chmin(T1 &a, T2 b) { return a > b && (a = b, true); }
// clang-format on

signed main(int argc, char *argv[]) {
  int n = in();
  int64_t C = in();

  vector<int64_t> x(n), v(n);
  for (int i = 0; i < n; ++i) {
    cin >> x[i] >> v[i];
  }

  int64_t ans = 0;

  auto f = [&]() {
    int64_t res = -infll;
    vector<int64_t> a, b, c;
    a = v;
    for (int i = 1; i < n; ++i) {
      a[i] += a[i - 1];
    }
    for (int i = 0; i < n; ++i) {
      a[i] -= x[i];
      chmax(res, a[i]);
    }

    b = a;
    for (int i = 0; i < n; ++i) {
      b[i] -= x[i];
    }

    c = v;
    for (int i = n - 2; i >= 0; --i) {
      c[i] += c[i + 1];
    }
    c.back() -= (C - x.back());
    for (int i = n - 2; i >= 0; --i) {
      c[i] -= (C - x[i]);
      chmax(c[i], c[i + 1]);
    }
    for (int i = 0; i + 1 < n; ++i) {
      chmax(res, b[i] + c[i + 1]);
    }

    return res;
  };

  chmax(ans, f());
  reverse(all(v));
  reverse(all(x));
  for (int i = 0; i < n; ++i) {
    x[i] = C - x[i];
  }
  chmax(ans, f());

  cout << ans << endl;
}


Measured memory_bytes: 4224