#pragma GCC diagnostic ignored "-Wunused-variable" 
#include <bits/stdc++.h>
using namespace std;

#define BOOST
#ifdef BOOST
#include <boost/multiprecision/cpp_int.hpp>
#include <boost/multiprecision/cpp_dec_float.hpp>
#include <boost/range/adaptor/reversed.hpp>
using namespace boost;
using ml = boost::multiprecision::cpp_int;
using md = boost::multiprecision::cpp_dec_float_100;
#define forir(i, ...) if(ll i=(ll)v.size())for(__VA_ARGS__)if(i--,1)
#define eachr(i, e, c) forir(i, auto &&e: adaptors::reverse(c))
#endif

/***** type *****/
using ll = long long;
using ld = long double;
using pll = pair<long long, long long>;
template <class T> using vt = vector<T>;
template <class T> using vvt = vector<vector<T>>;
template <class T> using vvvt = vector<vector<vector<T>>>;
/***** define *****/
#define all(c) (c).begin(), (c).end()            // begin to end
#define coutd cout << fixed << setprecision(10)  // cout double
#define output(x) do { cout << x << endl; return 0; } while(0)
#define rep(i, b, e) for (ll i = b; i < e; i++)  // repeat
#define repr(i, b, e) for (ll i = b; e < i; i--) // repeat reverse
#define fori(i, ...) if (ll i = -1) for(__VA_ARGS__) if (i++, 1)
#define each(i, e, c) fori (i, auto&& e: c)      // for each
/***** const value *****/
#define llong_max 9223372036854775807            // 9 * 10^18
#define ldbl_max 1.79769e+308                    // 1.7 * 10^308
#define pi 3.1415926535897932                    // 3.14 ...
/***** lambda *****/
auto Ceil = [] // if (a % b != 0) return a / b + 1;
(auto x) { return (ll)ceil(x); };
auto Count = [] // long long count value
(auto b, auto e, auto x) { return (ll)count(b, e, x); };
auto CtoL = [] // char to number
(auto c) { return (ll)c - (ll)'0'; };
auto LtoC = [] // number to char
(auto n) { return (char)('0' + n); };
auto Pow = [] // long long pow
(auto a, auto b) { return (ll)pow(a, b); };
auto Pow2 = [] // long long pow2
(auto n) { return (1LL << n); };
auto Pow10 = [] // long long pow10
(auto n) { return (ll)pow(10, n); };
auto Size = []  // long long collection size
(auto& c) { return (ll)(c).size(); };
auto Sum = [] // long long accumulate
(auto b, auto e) { return accumulate(b, e, 0LL); };

/***** template *****/
template <class T> void MakeVVT
(ll ys, ll xs, vvt<T>& v, T fill = T())
{	// vector<vector<T>> resize + fill
	v.resize(ys); rep(y, 0, ys) v[y].resize(xs, fill);
}
template <class T> void MakeVVVT
(ll zs, ll ys, ll xs, vvvt<T>& v, T fill = T())
{	// vector<vector<vector<T>>> resize + fill
	v.resize(zs); rep(z, 0, zs) MakeVVT(ys, xs, v[z], fill);
}
template <class T> void InputVT
(ll xs, vt<T>& v, T fix = T())
{	// input vector<T> (T != struct) + fix
	v.resize(xs); rep(i, 0, xs) { cin >> v[i]; v[i] += fix; }
}
template <class T> void InputVVT
(ll ys, ll xs, vvt<T>& v, T fix = T())
{	// input vector<vector<T>> (T != struct) + fix
	MakeVVT(ys, xs, v, fix);
	rep(y, 0, ys) rep(x, 0, xs) { cin >> v[y][x]; v[y][x] += fix; }
}
template <class T> void InputVVVT
(ll zs, ll ys, ll xs, vvvt<T>& v, T fix = T())
{	// input vector<vector<vector<T>>> (T != struct) + fix
	v.resize(zs); rep(z, 0, zs) InputVVT(ys, xs, v[z], fix);
}

/**************************************/
/********** BEGIN OF NYA LIB **********/
/**************************************/

namespace NyaGadget {}

namespace NyaGadget
{
	/***** 累積和ライブラリ *****/

	template <class T> struct DS_NyaaCumulativeSum
	{
		std::vector<T> sum;
		DS_NyaaCumulativeSum(std::vector<T>& v)
		{
			sum.push_back(v.front());
			for (long long i = 1; i < (long long)v.size(); ++i) sum.push_back(sum.back() + v[i]);
		}
		/**
		@brief 範囲の合計値を取得する
		@note
		 閉区間[s, t]の合計値を返す。
		 範囲外のsやtに対しては0を返す。
		**/
		T Range(long long s, long long t)
		{
			if (s < 0) s = 0;
			if ((long long)sum.size() <= t) t = (long long)sum.size() - 1;

			if (t < 0 || (long long)sum.size() <= s) return 0;
			return (s == 0) ? sum[t] : sum[t] - sum[s - 1];
		}
	};
}

/**************************************/
/*********** END OF NYA LIB ***********/
/**************************************/

using namespace NyaGadget;
//using mll = NT_ModLL< 1000000007 >;
//using mll = NT_ModLL< 998244353 >;

int main(void)
{
	ll N, C; cin >> N >> C; // f=x, s=v
	vt<pll> sushi(N); each(i, e, sushi) cin >> e.first >> e.second;
	N++;

	// 右回り往復
	vt<ll> rx(N); each(i, e, sushi) rx[i+1] = e.first;
	vt<ll> rv(N); each(i, e, sushi) rv[i+1] = e.second;
	DS_NyaaCumulativeSum csrv(rv);
	vt<ll> rmax(N);
	rep(i, 1, N) rmax[i] = max(rmax[i - 1], csrv.Range(0, i) - 2 * rx[i]);
	// 左回り
	vt<ll> lx(N); each(i, e, sushi) lx[i] = e.first; reverse(all(lx));
	vt<ll> lv(N); each(i, e, sushi) lv[i] = e.second; reverse(all(lv));
	DS_NyaaCumulativeSum cslv(lv);
	vt<ll> lmax(N);
	rep(i, 1, N) lmax[i] = max(lmax[i - 1], cslv.Range(0, i) - (C - lx[i]));
	// 右回り往復→左周り
	ll ans = 0;
	rep(l, 0, N) ans = max(ans, lmax[l] + rmax[N - 1 - l]);

	// 左回り往復
	vt<ll> lx2(N); each(i, e, sushi) lx2[i] = e.first; reverse(all(lx2));
	vt<ll> lv2(N); each(i, e, sushi) lv2[i] = e.second; reverse(all(lv2));
	DS_NyaaCumulativeSum cslv2(lv2);
	vt<ll> lmax2(N);
	rep(i, 1, N) lmax2[i] = max(lmax2[i - 1], cslv2.Range(0, i) - 2 * (C - lx2[i]));
	// 右回り
	vt<ll> rx2(N); each(i, e, sushi) rx2[i + 1] = e.first;
	vt<ll> rv2(N); each(i, e, sushi) rv2[i + 1] = e.second;
	DS_NyaaCumulativeSum csrv2(rv2);
	vt<ll> rmax2(N);
	rep(i, 1, N) rmax2[i] = max(rmax2[i - 1], csrv2.Range(0, i) - rx[i]);
	// 左周り往復→右回り
	rep(r, 0, N) ans = max(ans, rmax2[r] + lmax2[N - 1 - r]);

	cout << ans;
	return 0;
}


Measured memory_bytes: 17556