//include
//------------------------------------------
#include <vector>
#include <list>
#include <map>
#include <set>
#include <deque>
#include <queue>
#include <stack>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>

#include <sstream>
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <ctime>

#include <stdexcept>

using namespace std;

//conversion
//------------------------------------------
inline int toInt(string s) { int v; istringstream sin(s); sin >> v; return v; }
template<class T> inline string toString(T x) { ostringstream sout; sout << x; return sout.str(); }

//debug
#define DUMP(x)  cerr << #x << " = " << (x) << endl
#define DEBUG(x) cerr << #x << " = " << (x) << " (L" << __LINE__ << ")" << " " << __FILE__ << endl

// type alias
using ll = long long;
using ull = unsigned long long;

// constant value
const ll MOD = 1000000007LL;

template<class ll>
inline ll getCeilExponent(ll a)
{
	if (a <= 0)
	{
		throw invalid_argument("Vaule equal to or less than 0 should not be valid");
	}

	ll index = 0;
	--a;
	for (; a != 0; a >>= 1)
	{
		++index;
	}
	return index;
}

template<class ll>
inline ll getFloorExponent(ll a)
{
	if (a <= 0)
	{
		throw invalid_argument("Vaule equal to or less than 0 should not be valid");
	}

	ll index = 0;
	a >>= 1;
	for (; a != 0; a >>= 1)
	{
		++index;
	}
	return index;
}


map<ll, ull> getPrimeFactorization(ll num)
{
	map<ll, ull> primeToCount;

	int rootNum = sqrt(num);
	vector<bool> isCompositeNumber(rootNum + 1, false);
	for (int p = 2; num != 1 && p <= rootNum; ++p)
	{
		if (!isCompositeNumber[p])
		{
			for (int i = p * 2; i <= rootNum; i += p)
			{
				isCompositeNumber[i] = true;
			}

			int count = 0;
			while (num % p == 0)
			{
				++count;
				num /= p;
			}
			if (count >= 1)
			{
				primeToCount.insert(make_pair(p, count));
			}
		}
	}
	if (num != 1)
	{
		primeToCount.insert(make_pair(num, 1));
	}

	return primeToCount;
}

// Computational Complexity: O(log(max(a, b))) 
template<class ll>
inline ll getGCD(ll a, ll b)
{
	if (b == 0)
	{
		return a;
	}
	return getGCD(b, a % b);
}

// Computational Complexity: O(log(max(a, b))) 
template<class ll>
inline pair<ll, ll> getBezoutsIdentitySoulution(ll a, ll b)
{
	if (b == 0)
	{
		return { 1, 0 };
	}
	auto sol = getBezoutsIdentitySoulution(b, a % b);
	return { sol.second, sol.first - (a / b) * sol.second };
}

inline ll getModValue(ll a, ll m)
{
	return (a % m + m) % m;
}

// Computational Complexity: O(log(max(a, n))) 
inline ll getModInverse(ll a, ll m)
{
	auto sol = getBezoutsIdentitySoulution(a, m);
	if (a * sol.first + m * sol.second != 1)
	{
		return -1;
	}
	return getModValue(sol.first, m);
}

ll getModCombinatorialNumber(ll n, ll r, ll m)
{
	ll res = 1;
	for (; n > 0; --n, --r)
	{
		res = (((res * n) % m) * getModInverse(n, m)) % m;
	}
	return getModValue(res, m);
}

ll getModPower(ll base, unsigned t, ll m)
{
	if (t == 0)
	{
		return 1LL;
	}

	auto res = getModPower(base, t >> 1, m);
	res = getModValue(res * res, m);
	if (t % 2 == 1)
	{
		res = getModValue(res * base, m);
	}
	return res;
}

int main()
{
	cin.tie(0);
	ios::sync_with_stdio(false);

	ll n, c;
	cin >> n >> c;

	vector<pair<ll, ll>> sushis(n + 1);
	ll x, v;
	for (ll i = 1; i <= n; ++i)
	{
		cin >> x >> v;
		sushis[i] = {x, v};
	}

	ll lSum = 0;
	vector<ll> lSums(n + 1);
	vector<ll> lReturnMaxs(n + 1);
	ll rSum = 0;
	vector<ll> rSums(n + 1);
	vector<ll> rReturnMaxs(n + 1);
	for (ll i = 1; i <= n; ++i)
	{
		lSum += sushis[i].second;
		lSums[i] = lSum - sushis[i].first;
		lReturnMaxs[i] = max(lReturnMaxs[i - 1], lSum - sushis[i].first * 2);

		rSum += sushis[n + 1 - i].second;
		rSums[i] = rSum - ( c - sushis[n + 1 - i].first );
		rReturnMaxs[i] = max(rReturnMaxs[i - 1], rSum - (c - sushis[n + 1 - i].first) * 2);
	}

	ll res = 0;
	for (ll i = 1; i <= n; ++i)
	{
		res = max(res, lSums[i] + rReturnMaxs[n - i]);
		res = max(res, rSums[i] + lReturnMaxs[n - i]);
	}

	cout << res << endl;

	return 0;
}




Measured memory_bytes: 4992