#include <bits/stdc++.h>
#define REP(i, n) for (int i = 0; i < n; i++)
#define FOR(i, m, n) for (int i = m; i < n; i++)
#define ll long long int
const int INF = 2147483647;
const ll MOD = 1000000007;

using namespace std;

template <class T> class SegmentTree {
public:
  SegmentTree(const int n, const T id, function<T(T, T)> op,
              function<T(T, T)> updateOp);
  void update(const int index, const T value);
  T find(const int l, const int r);

private:
  int leftChildIndex(const int parent);
  int rightChildIndex(const int parent);
  int parent(const int child);
  int leaf(const int index);
  T query(const int l, const int r, const int node, const int al, const int ar);

private:
  vector<T> data;
  int leafNum;
  T id;
  function<T(T, T)> op;
  function<T(T, T)> updateOp;
};

template <class T>
SegmentTree<T>::SegmentTree(const int n, const T id, function<T(T, T)> op,
                            function<T(T, T)> updateOp)
    : id(id), op(op), updateOp(updateOp) {
  leafNum = 1;
  while (leafNum < n)
    leafNum *= 2;
  data = vector<T>(2 * leafNum - 1, id);
}

template <class T> int SegmentTree<T>::leftChildIndex(const int parent) {
  return parent * 2 + 1;
}

template <class T> int SegmentTree<T>::rightChildIndex(const int parent) {
  return parent * 2 + 2;
}

template <class T> int SegmentTree<T>::parent(const int child) {
  return (child - 1) / 2;
}

template <class T> int SegmentTree<T>::leaf(const int index) {
  return index + leafNum - 1;
}

// [l,r)で探す
template <class T> T SegmentTree<T>::find(const int l, const int r) {
  return query(l, r, 0, 0, leafNum);
}

template <class T>
T SegmentTree<T>::query(const int l, const int r, const int node, const int al,
                        const int ar) {
  if (r <= al || ar <= l)
    return id;
  if (l <= al && ar <= r)
    return data[node];
  int mid = (al + ar) / 2;
  T leftValue = query(l, r, leftChildIndex(node), al, mid);
  T rightValue = query(l, r, rightChildIndex(node), mid, ar);
  return op(leftValue, rightValue);
}

template <class T> void SegmentTree<T>::update(const int index, const T value) {
  int i = leaf(index);
  data[i] = updateOp(data[i], value);
  while (i > 0) {
    i = parent(i);
    data[i] = op(data[rightChildIndex(i)], data[leftChildIndex(i)]);
  }
}

int main() {
  int n;
  cin >> n;
  ll c;
  cin >> c;
  SegmentTree<ll> unClock(n, 0, [](ll a, ll b) -> ll { return max(a, b); },
                          [](ll a, ll b) -> ll { return max(a, b); });
  SegmentTree<ll> clock(n, 0, [](ll a, ll b) -> ll { return max(a, b); },
                        [](ll a, ll b) -> ll { return max(a, b); });
  vector<ll> cum(n + 1);
  cum[0] = 0;
  vector<ll> d(n);

  REP(i, n) {
    ll x, v;
    cin >> x >> v;
    d[i] = x;
    cum[i + 1] = cum[i] + v;
  }

  REP(i, n) {
    unClock.update(i, cum[i + 1] - d[i]);
    clock.update(i, cum[n] - cum[n - i - 1] - (c - d[n - i - 1]));
  }

  ll ans = max(clock.find(0, n), unClock.find(0, n));

  REP(i, n) {
    ll tmp = cum[i + 1] - (d[i] * 2);
    tmp += clock.find(0, n - i - 1);
    ans = max(ans, tmp);
  }

  REP(i, n) {
    ll tmp = cum[n] - cum[n - i - 1] - (c - d[n - i - 1]) * 2;
    tmp += unClock.find(0, n - i - 1);
    ans = max(ans, tmp);
  }

  cout << ans << endl;
}


Measured memory_bytes: 5888