#include <bits/stdc++.h>
using namespace std;
/*{{{*/  //template
#define rep(i,n) for(int i=0;i<(int)(n);i++)
constexpr int INF = numeric_limits<int>::max()/2;
constexpr long long LINF = numeric_limits<long long>::max()/3;
#define mp make_pair
#define pb push_back
#define eb emplace_back
#define fi first
#define se second
#define all(v) (v).begin(),(v).end()
#define sz(x) (int)(x).size()
#define debug(x) cerr<<#x<<":"<<x<<endl
#define debug2(x,y) cerr<<#x<<","<<#y":"<<x<<","<<y<<endl
//struct fin{ fin(){ cin.tie(0); ios::sync_with_stdio(false); } } fin_;
struct Double{ double d; explicit Double(double x) : d(x){} };
ostream& operator<<(ostream& os,const Double x){ os << fixed << setprecision(20) << x.d; return os; }
template<typename T> ostream& operator<<(ostream& os,const vector<T>& vec){ os << "["; for(const auto& v : vec){ os << v << ","; } os << "]"; return os; }
template<typename T,typename U> ostream& operator<<(ostream& os,const pair<T,U>& p){ os << "(" << p.first << ","<< p.second <<")"; return os; }
template<typename T> ostream& operator<<(ostream& os,const set<T>& st){ os<<"{"; for(T v:st) os<<v<<","; os <<"}"; return os; }
template<typename T,typename U> inline void chmax(T &x,U y){ if(y>x) x = y; }
template<typename T,typename U> inline void chmin(T &x,U y){ if(y<x) x = y; }
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int,int> pii;
typedef vector<int> vi;
typedef vector<vi> vvi;
ll gcd(ll a,ll b){ if(b==0) return a; else return gcd(b,a%b); }
//constexpr double eps = 1e-14; 
constexpr double eps = 1e-10; 
constexpr ll mod = 1e9+7;
const int dx[]={1,0,-1,0} ,dy[] = {0,1,0,-1};
/*}}}*/

ll N,C;
vector<ll> x,v;

ll lsumx[100005],rsumx[100005];
ll lsumv[100005],rsumv[100005];

ll lmax1[100005],rmax1[100005];
ll lmax2[100005],rmax2[100005];

int main(){
    cin >> N >> C;
    x.resize(N);
    v.resize(N);
    rep(i,N){
        cin >> x[i] >> v[i];
    }

    for(int i=N-1;i>=0;i--){
        lsumx[i] = C-x[i];
        lsumv[i] = lsumv[i+1] + v[i];
    }
    for(int i=0;i<N;i++){
        if(i==0){
            rsumx[i] = x[i];
            rsumv[i] = v[i];
        }else{
            rsumx[i] = x[i];
            rsumv[i] = rsumv[i-1] + v[i];
        }
    }

    for(int i=N-1;i>=0;i--){
        if(i==N-1){
            lmax1[i] = max(0ll,lsumv[i]-lsumx[i]);
            lmax2[i] = max(0ll,lsumv[i]-lsumx[i]*2);
        }else{
            lmax1[i] = max(lmax1[i],lsumv[i]-lsumx[i]);
            lmax2[i] = max(lmax2[i],lsumv[i]-lsumx[i]*2);
        }
    }


    for(int i=0;i<N;i++){
        if(i==0){
            rmax1[i] = max(0ll,rsumv[i]-rsumx[i]);
            rmax2[i] = max(0ll,rsumv[i]-rsumx[i]*2);
        }else{
            rmax1[i] = max(rmax1[i-1],rsumv[i]-rsumx[i]);
            rmax2[i] = max(rmax2[i-1],rsumv[i]-rsumx[i]*2);
        }
    }

    ll ans = 0;

    for(int i=0;i<N;i++){
        // left [N-1 --> i+1]
        // right [0 --> i]
        ll p1 = rmax1[i];
        ll p2 = lmax1[i+1];

        ll p3 = rmax1[i] + lmax2[i+1];
        ll p4 = rmax2[i] + lmax1[i+1];

        ll mx = max({p1,p2,p3,p4});
        ans=max(mx,ans);
    }


    ans=max(ans,rmax1[N-1]);
    ans=max(ans,lmax1[0]);


    assert(ans>=0);
    cout << ans << endl;

    /*
    cout << "lmax1" << endl;
    for(int i=0;i<N;i++){
        cout << i << " " << lmax1[i] << endl;
    }

    cout << "lsumx , v" << endl;
    rep(i,N){
        cout << i << " " << lsumx[i] << " " << lsumv[i] << endl;
    }
    */
}


Measured memory_bytes: 8064