#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef pair<ll, ll> P;
typedef pair<int, int> Pi;
typedef vector<ll> Vec;
typedef vector<int> Vi;
typedef vector<string> Vs;
typedef vector<P> VP;
typedef vector<vector<ll>> VV;
typedef vector<vector<int>> VVi;

#define REP(i, a, b) for(ll i=(a); i<(b); i++)
#define PER(i, a, b) for(ll i=(a); i>=(b); i--)
#define rep(i, n) REP(i, 0, n)
#define per(i, n) PER(i, n, 0)
const ll INF=1e18+18;
const ll MAX=100005;
const ll MOD=1000000007;
#define Yes(n) cout << ((n) ? "Yes" : "No") << endl;
#define YES(n) cout << ((n) ? "YES" : "NO") << endl;
#define ALL(v) v.begin(), v.end()
#define rALL(v) v.rbegin(), v.rend()
#define pb(x) push_back(x)
#define mp(a, b) make_pair(a,b)
#define Each(a,b) for(auto &a :b)
#define REPM(i, mp) for (auto i = mp.begin(); i != mp.end(); ++i)
#define dbg(x_) cerr << #x_ << ":" << x_ << endl;
#define dbgmap(mp) cerr << #mp << ":"<<endl; for (auto i = mp.begin(); i != mp.end(); ++i) { cerr << i->first <<":"<<i->second << endl;}
#define dbgarr(n,m,arr) rep(i,n){rep(j,m){cerr<<arr[i][j]<<" ";}cerr<<endl;}
#define dbgdp(n,arr) rep(i,n){cerr<<arr[i]<<" ";}cerr<<endl;
#define dbgmint(n,arr) rep(i,n){cerr<<arr[i].x<<" ";}cerr<<endl;
#define out(a) cout<<a<<endl
#define out2(a,b) cout<<a<<" "<<b<<endl
#define vout(v) rep(i,v.size()){cout<<v[i]<<" ";}cout<<endl
#define Uniq(v) v.erase(unique(v.begin(), v.end()), v.end())
#define fi first
#define se second

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

template<typename T1, typename T2>
ostream &operator<<(ostream &s, const pair<T1, T2> &p) { return s<<"("<<p.first<<", "<<p.second<<")"; }

template<typename T>istream& operator>>(istream&i,vector<T>&v)
{rep(j,v.size())i>>v[j];return i;}

// vector
template<typename T>
ostream &operator<<(ostream &s, const vector<T> &v) {
    int len=v.size();
    for(int i=0; i<len; ++i) {
        s<<v[i];
        if(i<len-1) s<<"	";
    }
    return s;
}

// 2 dimentional vector
template<typename T>
ostream &operator<<(ostream &s, const vector<vector<T> > &vv) {
    int len=vv.size();
    for(int i=0; i<len; ++i) {
        s<<vv[i]<<endl;
    }
    return s;
}

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    cout<<std::setprecision(10);

    ll n,c;
    cin>>n>>c;
    Vec x(n);
    Vec v(n);
    rep(i,n){
        cin>>x[i]>>v[i];
    }

    Vec r1(n);
    Vec r2(n);
    Vec l1(n);
    Vec l2(n);

    rep(i,n){
        r1 [i] = v[i]-x[i] + (i>0 ? x[i-1]:0);
        r2 [i] = v[i]-2*x[i]  +  (i>0 ? 2*x[i-1] :0);
        l1 [i] = v[n-1-i] - (c - x[n-1-i]) +  (i>0?(c - x[n-i]):0);
        l2 [i] = v[n-1-i] - 2 * (c - x[n-1-i]) + (i>0? 2 * (c - x[n-i]):0);
    }

    Vec sr1(n);
    Vec sr2(n);
    Vec sl1(n);
    Vec sl2(n);

    rep(i,n){
        sr1[i] = r1[i];
        sr2[i] = r2[i];
        sl1[i] = l1[i];
        sl2[i] = l2[i];
        if(i>0){
            sr1[i] += sr1[i-1];
            sr2[i] += sr2[i-1];
            sl1[i] += sl1[i-1];
            sl2[i] += sl2[i-1];
        }
    }

    Vec mr1(n);
    Vec mr2(n);
    Vec ml1(n);
    Vec ml2(n);
    rep(i,n){
        mr1[i] = max(sr1[i],0LL);
        mr2[i] = max(sr2[i],0LL);
        ml1[i] = max(sl1[i],0LL);
        ml2[i] =  max(sl2[i],0LL);
        if(i>0){
            chmax(mr1[i],mr1[i-1]);
            chmax(mr2[i],mr2[i-1]);
            chmax(ml1[i],ml1[i-1]);
            chmax(ml2[i],ml2[i-1]);
        }
    }

    ll ans = 0;
//    dbg(mr1);
//    dbg(mr2);
//    dbg(ml1);
//    dbg(ml2);
    rep(i,n){
        chmax(ans,mr1[i]+ (n-2-i>=0?ml2[n-2-i]:0) );
        chmax(ans,ml1[i]+ (n-2-i>=0?mr2[n-2-i]:0) );
    }
    cout<<ans<<endl;
    return  0;
}


Measured memory_bytes: 11264