#include <bits/stdc++.h>
using namespace std;
template <typename T>
ostream &operator<<(ostream &os, const vector<T> &v) {
  os << "{";
  for (int i = (int)(0); i < (int)(v.size()); ++i)
    os << v[i] << (i == (int)v.size() - 1 ? "" : ", ");
  os << "}";
  return os;
}
template <typename T, typename U>
ostream &operator<<(ostream &os, const pair<T, U> &p) {
  return (os << "(" << p.first << ", " << p.second << ")");
}
template <typename T, typename U>
ostream &operator<<(ostream &os, const map<T, U> &m) {
  bool first = true;
  os << "{";
  for (const auto &e : m) {
    if (!first) os << ", ";
    os << "{" << e.first << ": " << e.second << "}";
    first = false;
  }
  os << "}";
  return os;
}
template <typename T>
ostream &operator<<(ostream &os, const set<T> &s) {
  os << "{";
  bool first = true;
  for (const auto &e : s) {
    if (!first) os << ", ";
    os << e;
    first = false;
  }
  os << "}";
  return os;
}
template <typename T>
ostream &operator<<(ostream &os, const multiset<T> &s) {
  os << "{";
  bool first = true;
  for (const auto &e : s) {
    if (!first) os << ", ";
    os << e;
    first = false;
  }
  os << "}";
  return os;
}
template <typename T>
T dup(T x, T y) {
  return (x + y - 1) / y;
};
template <typename A, size_t N, typename T>
inline void arrayFill(A (&array)[N], const T &val) {
  std::fill((T *)array, (T *)(array + N), val);
}
template <class T>
inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T>
inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
struct in {
  const size_t n = 0;
  in() = default;
  in(size_t n) : n(n){};
  template <typename T>
  operator T() {
    T ret;
    cin >> ret;
    return ret;
  }
  template <typename T>
  operator vector<T>() {
    assert(n != 0);
    vector<T> ret(n);
    for (T &x : ret) {
      T tmp = in();
      x = tmp;
    }
    return ret;
  }
  template <typename T, typename U>
  operator pair<T, U>() {
    pair<T, U> ret;
    ret.first = in();
    ret.second = in();
    return ret;
  }
};
namespace fiore::impl {
template <typename T>
inline void out_impl(const T &x, char end_char) {
  std::cout << x << end_char;
}
template <typename T>
inline void out_impl(const vector<T> &x, char end_char) {
  bool first = true;
  for (const auto &e : x) {
    if (!first) std::cout << ' ';
    std::cout << e;
    first = false;
  }
  std::cout << end_char;
}
}  // namespace fiore::impl
template <typename T>
inline void out(const T &x) {
  fiore::impl::out_impl(x, '\n');
};
template <typename T, typename U, typename... Args>
inline void out(const T &x, const U &y, const Args &...args) {
  fiore::impl::out_impl(x, ' ');
  out(y, args...);
}
using ll = int64_t;
using vint = vector<int32_t>;
using vvint = vector<vint>;
using vll = vector<ll>;
using vvll = vector<vll>;
using vstr = vector<string>;
using pint = pair<int32_t, int32_t>;
using vpint = vector<pint>;
using pll = pair<ll, ll>;
using vpll = vector<pll>;
using setint = set<int32_t>;
using qint = queue<int32_t>;
using qpint = queue<pint>;
constexpr std::int32_t INF = 1001001001;
constexpr std::int64_t LINF = 1001001001001001001;
template <typename T>
class SegmentTree {
  using F = function<T(T, T)>;
  int n;
  const F f;
  const T ti;
  vector<T> dat;

 public:
  SegmentTree(const int n_, const F f, const T ti,
              const vector<T> &v = vector<T>())
      : f(f), ti(ti) {
    if (n_ == (int)v.size())
      build(n_, v);
    else
      init(n_);
  }
  void set_val(int k, const T x) {
    dat[k += n] = x;
    while (k >>= 1) dat[k] = f(dat[(k << 1) | 0], dat[(k << 1) | 1]);
  }
  T query(const int a, const int b) {
    if (a >= b) return ti;
    T vl = ti, vr = ti;
    for (int l = a + n, r = b + n; l < r; l >>= 1, r >>= 1) {
      if (l & 1) vl = f(vl, dat[l++]);
      if (r & 1) vr = f(dat[--r], vr);
    }
    return f(vl, vr);
  }
  T operator[](const int k) const { return dat[k + n]; }

 private:
  void init(const int n_) {
    n = 1;
    while (n < n_) n <<= 1;
    dat.assign(n << 1, ti);
  }
  void build(const int n_, const vector<T> &v) {
    init(n_);
    for (int i = 0; i < n_; i++) dat[n + i] = v[i];
    for (int i = n - 1; i; i--)
      dat[i] = f(dat[(i << 1) | 0], dat[(i << 1) | 1]);
  }
};
void Main() {
  int n = in();
  ll c = in();
  vll x(n), v(n);
  for (int i = (int)(0); i < (int)(n); ++i) cin >> x[i] >> v[i];
  vll s(n + 1, 0);
  for (int i = (int)(0); i < (int)(n); ++i) {
    s[i + 1] = s[i] + v[i];
  }
  vll v2(n), s2(n + 1, 0), s3(n + 1, 0);
  for (int i = (int)(0); i < (int)(n); ++i) {
    s2[i + 1] = s[i + 1] - (x[i] * 2);
    s3[n - i] = s[n] - s[i] - ((c - x[i]) * 2);
  };
  ;
  ;
  SegmentTree<ll> seg2(
      n + 1, [](ll a, ll b) { return max(a, b); }, -LINF, s2);
  SegmentTree<ll> seg3(
      n + 1, [](ll a, ll b) { return max(a, b); }, -LINF, s3);
  ll ans = 0;
  for (int i = (int)(0); i < (int)(n); ++i) {
    ll a = s[i + 1] - x[i];
    ll aa = a + seg3.query(0, n - i);
    chmax(ans, max(a, aa));
    ll b = (s[n] - s[i]) - (c - x[i]);
    ll bb = b + seg2.query(0, i);
    chmax(ans, max(b, bb));
  }
  out(ans);
}
signed main() {
  std::cin.tie(nullptr);
  std::ios_base::sync_with_stdio(false);
  std::cout << std::fixed << std::setprecision(15);
  Main();
  return 0;
}
