#include <iostream>
#include <cstdio>
#include <vector>
#define _USE_MATH_DEFINES
#include <math.h>
#include <cstring>
#include <numeric>
#include <algorithm>
#include <stdlib.h>
#include <functional>
#include <string>
#include <list>
#include <fstream>
#include <iomanip>
#include <array>
#include <map>
#include <queue>
#include <limits.h>
#include <set>
#include <stack>
#include <random>
#include <complex>
#include <unordered_map>
#include <unordered_set>
#include <chrono>
#include <bitset>
#define rep(i,s,n) for(int i = (s); (n) > i; i++)
#define REP(i,n) rep(i,0,n)
#define RANGE(x,a,b) ((a) <= (x) && (x) <= (b))
#define DUPLE(a,b,c,d) (RANGE(a,c,d) || RANGE(b,c,d) || RANGE(c,a,b) || RANGE(d,a,b))
#define INCLU(a,b,c,d) (RANGE(a,c,d) && (b,c,d))
#define PW(x) ((x)*(x))
#define ALL(x) (x).begin(), (x).end()
#define MODU 1000000007
#define bitcheck(a,b)   ((a >> b) & 1)
#define bitset(a,b)      ( a |= (1 << b))
#define bitunset(a,b)    (a &= ~(1 << b))
#define MP(a,b) make_pair((a),(b))
#define Manh(a,b) (abs((a).first-(b).first) + abs((a).second - ((b).second))
#define pritnf printf
#define scnaf scanf
#define itn int

#include <nmmintrin.h>
#ifdef _MSC_VER

#define __builtin_popcount _mm_popcnt_u32
#define __builtin_popcountll _mm_popcnt_u64
#endif
using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
template<typename A, size_t N, typename T>
void Fill(A(&array)[N], const T &val) {
	std::fill((T*)array, (T*)(array + N), val);
}

struct Edge {
	int from, to, w;
};


template<class T, class OP, class PROP>
class Lazy_Segment_Tree {//0-indexed 半開
public:
	vector<T> data, lazy;
	vector<bool> isupd;
	int n;
	Lazy_Segment_Tree(int a) :n(1) {
		while (n < a) n *= 2;
		data.resize(n * 2 - 1);
		lazy.resize(n * 2 - 1);
		isupd.resize(n * 2 - 1);
	}

	void prop(int num, int s) {
		if (isupd[num]) {
			data[num] = PROP()(data[num], lazy[num], 1, s);
			if (num < n - 1) {
				lazy[num * 2 + 1] = PROP()(lazy[num * 2 + 1], lazy[num]);
				lazy[num * 2 + 2] = PROP()(lazy[num * 2 + 2], lazy[num]);
				isupd[num * 2 + 1] = 1;
				isupd[num * 2 + 2] = 1;
			}
			lazy[num] = T();
			isupd[num] = 0;
		}
	}
	void op(int a, int b, T v, int num = 0, int base = 1) {
		int l = (num + 1 - base) * (n / base), r = l + n / base;
		prop(num, r - l);
		if (a == l && b == r) {
			isupd[num] = 1;
			lazy[num] = PROP()(lazy[num], v);
			prop(num, r - l);
		}
		else {
			int nr = (l + r) / 2;
			if (nr > a) op(a, min(b, nr), v, num * 2 + 1, base * 2);
			if (nr < b) op(max(a, nr), b, v, num * 2 + 2, base * 2);
			prop(num * 2 + 1, (r - l) / 2), prop(num * 2 + 2, (r - l) / 2);
			data[num] = OP()(data[num * 2 + 1], data[num * 2 + 2]);
		}
	}

	T query(int a, int b, int num = 0, int base = 1) {
		int l = (num + 1 - base) * (n / base), r = l + n / base;

		prop(num, r - l);

		if (a == l && b == r)
			return data[num];
		else {
			int nr = (l + r) / 2;
			T ret = T();
			if (nr > a) ret = OP()(ret, query(a, min(b, nr), num * 2 + 1, base * 2));
			if (nr < b) ret = OP()(ret, query(max(a, nr), b, num * 2 + 2, base * 2));
			return ret;
		}
	}
};


template<class T>
class ADD {
public:
	T operator ()(const T a, const T b, const int ra = 1, int rb = 1) {
		return a*ra + b*rb;
	}
};

template<class T>
class UPD {
public:
	T operator ()(const T a, const T b, const int ra = 1, int rb = 1) {
		return b;
	}
};

template<class T>
class RMQ {
public:
	T operator ()(const T a, const T b) {
		return max(a, b);
	}
};
template<class T>
class SUM {
public:
	T operator ()(const T a, const T b) {
		return a + b;
	}
};

struct N {
public:
	ll num;
	N(ll num) : num(num) {}
	N() :num(0) {}
	N operator + (const N obj) const {
		return N(obj.num + num);
	}
	N operator * (const N obj) const {
		return N(obj.num * num);
	}
	bool operator < (const N obj)const {
		return obj.num > num;
	}
};


ll solve(int n, ll c, vector<ll> x, vector<ll> v) {

	vector<ll> rui(n + 1), js(n + 1);
	ll bp = 0;
	REP(i, n) {
		rui[i + 1] = rui[i] + v[i] - (x[i] - bp);
		js[i + 1] = js[i] + v[i];
		bp = x[i];
	}

	Lazy_Segment_Tree<ll, RMQ<ll>, UPD<ll>> seg(n);
	REP(i, n) {
		seg.op(i, i + 1, rui[i + 1]);
	}
	ll ans = 0;
	ll csum = js[n];
	REP(i, n) {
		ll clen = c - x[i];
		ll cans = csum - clen;

		ans = max(ans, cans);

		cans -= clen;

		cans += seg.query(0, i);

		ans = max(ans, cans);

		csum -= v[i];
	}

	ans = max(ans, seg.query(0, n));



	return ans;
}


int main() {

	int n;
	ll c;
	cin >> n >> c;

	vector<ll> x(n), v(n);

	REP(i, n) {
		scanf("%lld %lld", &x[i], &v[i]);
	}
	ll ans = 0;
	ans = max(ans, solve(n,c,x,v));
	REP(i, n) {
		x[i] = c - x[i];
	}
	reverse(ALL(x));
	reverse(ALL(v));
	ans = max(ans, solve(n, c, x, v));
	cout << ans << endl;

	return 0;
}

Measured memory_bytes: 9088