#include <iostream>
#include <algorithm>
#include <utility>
#include <string>
#include <vector>
#include <set>
#include <stack>
#include <queue>
#include <map>
#include <math.h>
#include <string.h>
#include <iomanip>
#include <numeric>
#include <cstdlib>
#include <cstdint>
#include <cmath>
#include <functional>
#include <limits>
#include <cassert>

using namespace std;

/* template */

using ll = long long;

void debug_out() { std::cout << std::endl; }
 
template <typename Head, typename... Tail>
void debug_out(Head H, Tail... T) {
  cout << H << " ";
  debug_out(T...);
}

#ifdef LOCAL
#define debug(...) cout << "debug: "; debug_out(__VA_ARGS__)
#else
#define debug(...) 
#endif

#define rep(i, a, n) for (int i = (int)(a); i < (int)(n); i++)
#define rrep(i, a, n) for (int i = ((int)(n-1)); i >= (int)(a); i--)
#define Rep(i, a, n) for (long long i = (long long)(a); i< (long long)(n); i++)
#define RRep(i, a, n) for (long long i = ((long long)(n-1ll)); i>=(long long)(a); i--)
#define all(v) (v).begin(),(v).end()
#define rall(v) (v).rbegin(),(v).rend()

template <typename T>
std::ostream& operator<<(std::ostream& os, std::vector<T> vec) {
  for (std::size_t i = 0; i < vec.size(); i++)os << vec[i] << (i + 1 == vec.size() ? "" : " ");
  return os;
}

struct Edge{
    int to;
    ll weight;
    Edge(int t, ll w) : to(t), weight(w){ }
};

struct edge{
    int from;
    int to;
    ll weight;
    edge(int f,int t,ll w) : from(f), to(t), weight(w){ }
};


using Graph = vector<vector<Edge>>;
using graph = vector<vector<int>>;
using edges = vector<edge>;

template <class T>
inline bool chmin(T &a, T b){
    if (a > b){
        a = b;
        return true;
    }
    return false;
}

template <class T>
inline bool chmax(T &a, T b){
    if (a < b){
        a = b;
        return true;
    }
    return false;
}

template <class T>
T lcm(T a, T b){
    return (a*b)/gcd(a,b);
}

bool isPalindrome(string s){
    string t = s;
    reverse(all(t));
    return s==t;
}

constexpr ll LNF = 1e15;

constexpr int INF = 1e9+7;

vector<int> dx = {-1,0,1,0,-1,1,-1,1};
vector<int> dy = {0,-1,0,1,1,-1,-1,1};

/* template */

int main(){
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    ll n,c; cin >> n >> c;
    vector<pair<ll,ll>> x(n);
    rep(i,0,n) cin >> x[i].first >> x[i].second;
    vector<pair<ll,ll>> right(n+1,make_pair(0,0));
    vector<pair<ll,ll>> left(n+1,make_pair(0,0));
    pair<ll,ll> flag = {0,0};
    rep(i,0,n){
        if(i==0){
            flag.first = x[i].second - x[i].first;
            flag.second = -c+x[n-1-i].first + x[n-1-i].second;
        } else{
            flag.first += x[i].second - x[i].first + x[i-1].first;
            flag.second += x[n-1-i].second - x[n-i].first + x[n-1-i].first;
        }
        right[i+1] = right[i];
        left[i+1] = left[i];
        chmax(right[i+1].first,flag.first);
        chmax(left[i+1].first,flag.second);
        chmax(right[i+1].second,flag.first-x[i].first);
        chmax(left[i+1].second,flag.second-(c-x[n-1-i].first));
    }
    ll ans = 0ll;
    rep(i,0,n+1){
        ll p = right[i].first + left[n-i].second;
        ll q = right[i].second + left[n-i].first;
        chmax(ans,p);
        chmax(ans,q);
    }
    rep(i,0,n+1){
        debug(right[i].first,right[i].second,left[i].first,left[i].second);
    }
    cout << ans << endl;
}

Measured memory_bytes: 4992