// need
#include <iostream>
#include <algorithm>

// data structure
#include <bitset>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <utility>
#include <vector>
#include <complex>
//#include <deque>
#include <valarray>

// stream
//#include <istream>
//#include <sstream>
//#include <ostream>
#include <fstream>

// etc
#include <cassert>
#include <cmath>
#include <functional>
#include <iomanip>
#include <chrono>
#include <random>
#include <numeric>

// input
#define INIT std::ios::sync_with_stdio(false);std::cin.tie(0);
#define VAR(type, ...)type __VA_ARGS__;MACRO_VAR_Scan(__VA_ARGS__);
template<typename T> void MACRO_VAR_Scan(T& t) { std::cin >> t; }
template<typename First, typename...Rest>void MACRO_VAR_Scan(First& first, Rest&...rest) { std::cin >> first; MACRO_VAR_Scan(rest...); }
#define VEC_ROW(type, n, ...)std::vector<type> __VA_ARGS__;MACRO_VEC_ROW_Init(n, __VA_ARGS__); for(int i=0; i<n; ++i){MACRO_VEC_ROW_Scan(i, __VA_ARGS__);}
template<typename T> void MACRO_VEC_ROW_Init(int n, T& t) { t.resize(n); }
template<typename First, typename...Rest>void MACRO_VEC_ROW_Init(int n, First& first, Rest&...rest) { first.resize(n); MACRO_VEC_ROW_Init(n, rest...); }
template<typename T> void MACRO_VEC_ROW_Scan(int p, T& t) { std::cin >> t[p]; }
template<typename First, typename...Rest>void MACRO_VEC_ROW_Scan(int p, First& first, Rest&...rest) { std::cin >> first[p]; MACRO_VEC_ROW_Scan(p, rest...); }
#define VEC(type, c, n) std::vector<type> c(n);for(auto& i:c)std::cin>>i;
#define MAT(type, c, m, n) std::vector<std::vector<type>> c(m, std::vector<type>(n));for(auto& r:c)for(auto& i:r)std::cin>>i;

// output
#define OUT(d) std::cout<<(d);
#define FOUT(n, d) std::cout<<std::fixed<<std::setprecision(n)<<(d);
#define SOUT(n, c, d) std::cout<<std::setw(n)<<std::setfill(c)<<(d);
#define SP std::cout<<" ";
#define TAB std::cout<<"\t";
#define BR std::cout<<"\n";
#define SPBR(i, n) std::cout<<(i + 1 == n ? '\n' : ' ');
#define ENDL std::cout<<std::endl;
#define FLUSH std::cout<<std::flush;
#define SHOW(d) {std::cerr << #d << "\t:" << (d) << "\n";}
#define SHOWVECTOR(v) {std::cerr << #v << "\t:";for(const auto& xxx : v){std::cerr << xxx << " ";}std::cerr << "\n";}
#define SHOWVECTOR2(v) {std::cerr << #v << "\t:\n";for(const auto& xxx : v){for(const auto& yyy : xxx){std::cerr << yyy << " ";}std::cerr << "\n";}}
#define SHOWQUEUE(a) {auto tmp(a);std::cerr << #a << "\t:";while(!tmp.empty()){std::cerr << tmp.front() << " ";tmp.pop();}std::cerr << "\n";}

// utility
#define ALL(a) (a).begin(),(a).end()
#define FOR(i, a, b) for(int i=(a);i<(b);++i)
#define RFOR(i, a, b) for(int i=(b)-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 FORLL(i, a, b) for(ll i=ll(a);i<ll(b);++i)
#define RFORLL(i, a, b) for(ll i=ll(b)-1;i>=ll(a);--i)
#define REPLL(i, n) for(ll i=0;i<ll(n);++i)
#define RREPLL(i, n) for(ll i=ll(n)-1;i>=0;--i)
#define IN(a, x, b) (a<=x && x<b)
template<typename T> inline T CHMAX(T& a, const T b) { return a = (a < b) ? b : a; }
template<typename T> inline T CHMIN(T& a, const T b) { return a = (a > b) ? b : a; }
#define EXCEPTION(msg) throw std::string("Exception : " msg " [ in ") + __func__ + " : " + std::to_string(__LINE__) + " lines ]"
#define TRY(cond, msg) try {if (cond) EXCEPTION(msg);}catch (std::string s) {std::cerr << s << std::endl;}
void CHECKTIME(std::function<void()> f) { auto start = std::chrono::system_clock::now(); f(); auto end = std::chrono::system_clock::now(); auto res = std::chrono::duration_cast<std::chrono::nanoseconds>((end - start)).count(); std::cerr << "[Time:" << res << "ns  (" << res / (1.0e9) << "s)]\n"; }

// test
template<class T> std::vector<std::vector<T>> VV(int n, int m, T init = T()) {
	return std::vector<std::vector<T>>(n, std::vector<T>(m, init));
}
template<typename S, typename T>
std::ostream& operator<<(std::ostream& os, std::pair<S, T> p) {
	os << "(" << p.first << ", " << p.second << ")"; return os;
}

// type/const
#define int ll
using ll = long long;
using ull = unsigned long long;
using ld = long double;
using PAIR = std::pair<int, int>;
using PAIRLL = std::pair<ll, ll>;
constexpr int INFINT = 1 << 30;                          // 1.07x10^ 9
constexpr int INFINT_LIM = (1LL << 31) - 1;              // 2.15x10^ 9
constexpr ll INFLL = 1LL << 60;                          // 1.15x10^18
constexpr ll INFLL_LIM = (1LL << 62) - 1 + (1LL << 62);  // 9.22x10^18
constexpr double EPS = 1e-9;
constexpr int MOD = 1000000007;
constexpr double PI = 3.141592653589793238462643383279;

template<class T, size_t N> void FILL(T(&a)[N], const T& val) { for (auto& x : a) x = val; }
template<class ARY, size_t N, size_t M, class T> void FILL(ARY(&a)[N][M], const T& val) { for (auto& b : a) FILL(b, val); }
template<class T> void FILL(std::vector<T>& a, const T& val) { for (auto& x : a) x = val; }
template<class ARY, class T> void FILL(std::vector<std::vector<ARY>>& a, const T& val) { for (auto& b : a) FILL(b, val); }

// ------------>8------------------------------------->8------------

// ------------>8------------ SegmentTree ------------>8------------

// -- Query Type enum.
enum QUERY { SUM, MAX, MIN };
template <class T = int>
class SegmentTree {
private:
	int ARY_SIZE;
	T init_val;
	std::vector<T> ary;
	std::function<T(T, T)> func;
public:
	SegmentTree(int n, QUERY qtype) : ARY_SIZE(1) { init(n, qtype); }
	SegmentTree(int n, T init_val, QUERY qtype) : ARY_SIZE(1) { init(n, init_val, qtype); }
	void init(int n, QUERY qtype) {
		T init_val;
		switch (qtype) {
			case SUM: init_val = 0; break;
			case MAX: init_val = -INFLL; break;
			case MIN: init_val = std::numeric_limits<T>::max(); break;
			default: struct INVALID_QUERY_TYPE_ERROR {}; throw INVALID_QUERY_TYPE_ERROR(); break;
		}
		init(n, init_val, qtype);
	}
	void init(int n, T init_val, QUERY qtype) {
		std::function<T(T, T)> f;
		switch (qtype) {
			case SUM: f = [](T l, T r) {return l + r; }; break;
			case MAX: f = [](T l, T r) {return std::max(l, r); }; break;
			case MIN: f = [](T l, T r) {return std::min(l, r); }; break;
			default: struct INVALID_QUERY_TYPE_ERROR {}; throw INVALID_QUERY_TYPE_ERROR(); break;
		}
		init(n, init_val, f);
	}
	void init(int n, T init_val, std::function<T(T, T)> f) {
		while (ARY_SIZE < n) ARY_SIZE <<= 1;
		ARY_SIZE = ARY_SIZE * 2 - 1;
		ary.resize(ARY_SIZE, init_val);
		this->init_val = init_val;
		func = f;
	}
	// -- a[i] = val;
	void update(int i, T val) {
		i += ARY_SIZE / 2;
		ary[i] = val;
		while (i > 0) {
			i = (i - 1) / 2;
			ary[i] = func(ary[2 * i + 1], ary[2 * i + 2]);
		}
	}
	// -- a[i] += val;
	void add(int i, T val) {
		i += ARY_SIZE / 2;
		ary[i] += val;
		while (i > 0) {
			i = (i - 1) / 2;
			ary[i] = func(ary[2 * i + 1], ary[2 * i + 2]);
		}
	}
	// -- 区間[a, b)のfuncの返す値を返す (query_(a, b, 0, 0, n)と呼び出す)
	T query_(int a, int b, int i, int l, int r) {
		if (r <= a || b <= l) return init_val;
		if (a <= l && r <= b) return ary[i];
		else {
			T vl = query_(a, b, 2 * i + 1, l, (l + r) / 2);
			T vr = query_(a, b, 2 * i + 2, (l + r) / 2, r);
			return func(vl, vr);
		}
	}
	// -- [a, b)
	T query(int a, int b) { return query_(a, b, 0, 0, ARY_SIZE / 2 + 1); }
	T operator[](int i) { return ary[i + ARY_SIZE / 2]; }

	void debug_show() {
		for (int i = ARY_SIZE / 2; i < ARY_SIZE; ++i) std::cerr << ary[i] << " ";
		std::cerr << "\n";
	}
};
using ST = SegmentTree<int>;
// ------------8<------------ SegmentTree ------------8<------------

int solve(int n, int c, std::vector<int> x, std::vector<int> v) {
	ST st(n, MAX);

	int res = 0;
	int now = 0;
	REP(i, n) {
		now += v[i];
		st.update(i, now - x[i]);
	}
	CHMAX(res, st.query(0, n));

	now = 0;
	RREP(i, n) {
		now += v[i];
		CHMAX(res, st.query(0, i) - 2 * (c - x[i]) + now);
	}
	return res;
}

signed main() {
	INIT;

	VAR(int, n, c);
	VEC_ROW(int, n, x, v);

	int ans = 0;
	CHMAX(ans, solve(n, c, x, v));

	{
		auto X(x), V(v);
		REP(i, n) {
			x[i] = c - X[n - i - 1];
			v[i] = V[n - i - 1];
		}
	}
	CHMAX(ans, solve(n, c, x, v));
	OUT(ans)BR;
	return 0;
}


Measured memory_bytes: 5504