//include
//------------------------------------------
#include <bits/stdc++.h>
using namespace std;

//typedef
//------------------------------------------
typedef long long LL;
typedef vector<LL> VL;
typedef vector<VL> VVL;
typedef vector<string> VS;
typedef pair<LL, LL> PLL;

//container util
//------------------------------------------
#define ALL(a)  (a).begin(),(a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define PB push_back
#define MP make_pair
#define SZ(a) int((a).size())
#define EACH(i,c) for(typeof((c).begin()) i=(c).begin(); i!=(c).end(); ++i)
#define EXIST(s,e) ((s).find(e)!=(s).end())
#define SORT(c) sort((c).begin(),(c).end())

//constant
//--------------------------------------------
const double EPS = 1e-10;
const double PI  = acos(-1.0);
const int MOD = 1000000007;

// grid
//--------------------------------------------
VL dx = {0, 1, 0, -1};
VL dy = {1, 0, -1, 0};
VL dx2 = {-1, 0, 1, -1, 1, -1, 0, 1};
VL dy2 = {-1, -1, -1, 0, 0, 1, 1, 1};

//debug
//--------------------------------------------
#define dump(x)  cerr << #x << " = " << (x) << endl;
#define debug(x) cerr << #x << " = " << (x) << " (L" << __LINE__ << ")" << " " << __FILE__ << endl;

//IO accelerate
//--------------------------------------------
struct InitIO {
    InitIO() {
        cin.tie(nullptr);
        ios_base::sync_with_stdio(false);
        cout << fixed << setprecision(30);
    }
} init_io;

//template
//--------------------------------------------
template<typename T>
istream& operator >>(istream& is, vector<T>& vec) {
  for(T& x: vec) is >> x;
  return is;
}
template<typename T>
ostream& operator <<(ostream& os, const vector<T>& vec) {
  for(int i=0; i<vec.size(); i++){
    os << vec[i] << ( i+1 == vec.size() ? "" : "\t" );
  }
  return os;
}
template<typename T>
ostream& operator <<(ostream& s, const vector<vector<T>>& vv) {
	for (int i = 0; i < vv.size(); ++i) {
		s << vv[i] << endl;
	}
	return s;
}

// 多重vector
// auto dp=make_v<int>(4,h,w) みたいに使える
template<typename T>
vector<T> make_v(size_t a){return vector<T>(a);}

template<typename T,typename... Ts>
auto make_v(size_t a,Ts... ts){
  return vector<decltype(make_v<T>(ts...))>(a,make_v<T>(ts...));
}

// 多重vectorのためのfill
// fill_v(dp,0) みたいに使える
template<typename T,typename V>
typename enable_if<is_class<T>::value==0>::type
fill_v(T &t,const V &v){t=v;}

template<typename T,typename V>
typename enable_if<is_class<T>::value!=0>::type
fill_v(T &t,const V &v){
  for(auto &e:t) fill_v(e,v);
}

//main code

int main(int argc, char *argv[])
{
	LL n,c;
	cin >> n >> c;
	vector<PLL> xv(n+2);
	VL x(n), v(n);
	xv[0] = {0,0}; // 初期位置
	xv[n+1] = {c, 0};
	for (int i = 0; i < n; ++i) {
		cin >> x[i] >> v[i];
		xv[i+1] = (make_pair(x[i], v[i]));
	}
	VL sumv(n+1,0);
	VL sumvr(n+1,0);
	for (int i = 0; i < n; ++i) {
		sumv[i+1] = sumv[i] + xv[i+1].second;
		sumvr[i+1] = sumvr[i] + xv[n-i].second;
	}
	VL maxv(n+1,0);
	VL maxvr(n+1,0);
	for (int i = 0; i < n; ++i) {
		maxv[i+1] = max(maxv[i], sumv[i+1] - xv[i+1].first);
		maxvr[i+1] = max(maxvr[i], sumvr[i+1] - (c - xv[n-i].first));
	}
	LL ans = 0;
	for (int i = 0; i <= n; ++i) {
		// 2OA + OB
		ans = max(ans,sumv[i] - xv[i].first * 2 + maxvr[n-i]);
		// OA + 2OB
		ans = max(ans,sumvr[i] - (c - xv[n+1-i].first) * 2 + maxv[n-i]);
	}
	cout << ans << endl;
	return 0;
}


Measured memory_bytes: 6528