#include <iostream>
#include <algorithm>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <vector>
#include <queue>
#include <cmath>
#include <bitset>
#include <complex>
#include <functional>

// output
#define SPBR(w, n) std::cout<<(w + 1 == n ? '\n' : ' ');
#define YES cout << "YES" << endl
#define Yes cout << "Yes" << endl
#define NO cout << "NO" << endl
#define No cout << "No" << endl
// utility
#define ALL(i) (i).begin(), (i).end()
#define FOR(i, a, n) for(int i=(a);i<(n);++i)
#define RFOR(i, a, n) for(int i=(n)-1;i>=(a);--i)
#define REP(i, n) for(int i=0;i<int(n);++i)
#define RREP(i, n) for(int i=int(n)-1;i>=0;--i)
#define IN(a, x, b) (a<=x && x<b)
#define OUT(a, x, b) (x<a || b<=x)
template<class T> inline T chmax(T & a, const T b) { return a = (a < b) ? b : a; }
template<class T> inline T chmin(T& a, const T b) { return a = (a > b) ? b : a; }

// type/const
#define int ll
using ll = long long;
using ull = unsigned long long;
using ld = long double;
const int MOD = 1000000007;
/* const int MOD = 1000000007; */
const int INF = 1e18;
const double PI = acos(-1);

using namespace std;

template<typename Monoid>
struct SegmentTree {
  using F = function<Monoid(Monoid, Monoid)>;
  int sz;
  vector<Monoid> seg;
  const F f;
  const Monoid M1;
  // SegmentTree<int> seg(N, [](int a, int b) { return min(a, b); }, INT_MAX);
  SegmentTree(int n, const F f, const Monoid &M1) : f(f), M1(M1) {
    sz = 1;
    while(sz < n) sz <<= 1;
    seg.assign(2 * sz, M1);
  }
  // k番目にxを代入
  void set(int k, const Monoid &x) {
    seg[k + sz] = x;
  }
  // 構築
  void build() {
    for(int k = sz - 1; k > 0; k--) {
      seg[k] = f(seg[2 * k + 0], seg[2 * k + 1]);
    }
  }
  // k番目をxに変更
  void update(int k, const Monoid &x) {
    k += sz;
    seg[k] = x;
    while(k >>= 1) {
      seg[k] = f(seg[2 * k + 0], seg[2 * k + 1]);
    }
  }
  // [a, b)に対して結果を返す
  Monoid query(int a, int b) {
    Monoid L = M1, R = M1;
    for(a += sz, b += sz; a < b; a >>= 1, b >>= 1) {
      if(a & 1) L = f(L, seg[a++]);
      if(b & 1) R = f(seg[--b], R);
    }
    return f(L, R);
  }
  // k番目の要素を返す
  Monoid operator[](const int &k) const {
    return seg[k + sz];
  }

  template< typename C >
  int find_subtree(int a, const C &check, Monoid &M, bool type) {
    while(a < sz) {
      Monoid nxt = type ? f(seg[2 * a + type], M) : f(M, seg[2 * a + type]);
      if(check(nxt)) a = 2 * a + type;
      else M = nxt, a = 2 * a + 1 - type;
    }
    return a - sz;
  }
  // [a, x)がcheckを満たす最初の要素位置x
  template<typename C>
  int find_first(int a, const C &check) {
    Monoid L = M1;
    if(a <= 0) {
      if(check(f(L, seg[1]))) return find_subtree(1, check, L, false);
      return -1;
    }
    int b = sz;
    for(a += sz, b += sz; a < b; a >>= 1, b >>= 1) {
      if(a & 1) {
        Monoid nxt = f(L, seg[a]);
        if(check(nxt)) return find_subtree(a, check, L, false);
        L = nxt;
        ++a;
      }
    }
    return -1;
  }
  // [x, b)がcheckを満たす最後の要素位置x
  template<typename C>
  int find_last(int b, const C &check) {
    Monoid R = M1;
    if(b >= sz) {
      if(check(f(seg[1], R))) return find_subtree(1, check, R, true);
      return -1;
    }
    int a = sz;
    for(b += sz; a < b; a >>= 1, b >>= 1) {
      if(b & 1) {
        Monoid nxt = f(seg[--b], R);
        if(check(nxt)) return find_subtree(b, check, R, true);
        R = nxt;
      }
    }
    return -1;
  }
};
/* SegmentTree<int> seg(N, [](int a, int b) { return min(a, b); }, INF); */

signed main() {
  int N, C;
  cin >> N >> C;

  vector<int> X(N), V(N);
  REP(i, N) cin >> X[i] >> V[i];

  SegmentTree<int> ans1(N, [](int a, int b) { return max(a, b); }, -INF);
  SegmentTree<int> ans2(N, [](int a, int b) { return max(a, b); }, -INF);
  int ret = 0;
  int ans = 0;
  REP(i, N){
    ret += V[i];
    ans1.set(i, ret-X[i]);
    ans2.set(i, ret-2*X[i]);
    chmax(ans, ret-X[i]);
  }
  ans1.build();
  ans2.build();


  ret = 0;
  RREP(i, N){
    ret += V[i];
    chmax(ans, ret-(C-X[i]));
    chmax(ans, ret-2*(C-X[i])+ans1.query(0, i));
    chmax(ans, ret-(C-X[i])+ans2.query(0, i));
  }

  cout << ans << endl;

  return 0;
}

Measured memory_bytes: 5888