#include <bits/stdc++.h>
using namespace std;
#define int long long
#define rep(i,n) for(int (i)=0;(i)<(n);(i)++)
#define rrep(i,n) for(int (i)=((n)-1);(i)>=0;(i)--)
#define itn int
#define miele(v) min_element(v.begin(), v.end())
#define maele(v) max_element(v.begin(), v.end())
#define SUM(v) accumulate(v.begin(), v.end(), 0LL)
#define lb(a, key) lower_bound(a.begin(),a.end(),key)
#define ub(a, key) upper_bound(a.begin(),a.end(),key)
#define COUNT(a, key) count(a.begin(), a.end(), key) 
#define BITCOUNT(x) __builtin_popcount(x)
#define pb push_back
#define all(x) (x).begin(),(x).end()
#define F first
#define S second
using P = pair <int,int>;
using WeightedGraph = vector<vector <P>>;
using UnWeightedGraph = vector<vector<int>>;
using Real = long double;
using Point = complex<Real>; //Point and Vector2d is the same!
using Vector2d = complex<Real>;
const long long INF = 1LL << 60;
const int MOD = 1000000007;
const double EPS = 1e-15;
const double PI=3.14159265358979323846;
template <typename T> 
int getIndexOfLowerBound(vector <T> &v, T x){
    return lower_bound(v.begin(),v.end(),x)-v.begin();
}
template <typename T> 
int getIndexOfUpperBound(vector <T> &v, T x){
    return upper_bound(v.begin(),v.end(),x)-v.begin();
}
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; }
#define DUMPOUT cerr
#define repi(itr, ds) for (auto itr = ds.begin(); itr != ds.end(); itr++)
istream &operator>>(istream &is, Point &p) {
    Real a, b; is >> a >> b; p = Point(a, b); return is;
}
template <typename T, typename U>
istream &operator>>(istream &is, pair<T,U> &p_var) {
    is >> p_var.first >> p_var.second;
    return is;
}
template <typename T>
istream &operator>>(istream &is, vector<T> &vec) {
    for (T &x : vec) is >> x;
    return is;
}
template <typename T, typename U>
ostream &operator<<(ostream &os, pair<T, U> &pair_var) {
    DUMPOUT<<'{';
    os << pair_var.first;
    DUMPOUT<<',';
    os << " "<< pair_var.second;
    DUMPOUT<<'}';
    return os;
}
template <typename T>
ostream &operator<<(ostream &os, vector<T> &vec) {
    DUMPOUT<<'[';
    for (int i = 0; i < vec.size(); i++) 
    os << vec[i] << (i + 1 == vec.size() ? "" : " ");
    DUMPOUT<<']';
    return os;
}
template <typename T>
ostream &operator<<(ostream &os, vector<vector<T>> &df) {
  for (auto& vec : df) os<<vec;
  return os;
}
template <typename T, typename U>
ostream &operator<<(ostream &os, map<T, U> &map_var) {
    DUMPOUT << "{";
    repi(itr, map_var) {
        os << *itr;
        itr++;
        if (itr != map_var.end()) DUMPOUT << ", ";
        itr--;
    }
    DUMPOUT << "}";
    return os;
}
template <typename T>
ostream &operator<<(ostream &os, set<T> &set_var) {
    DUMPOUT << "{";
    repi(itr, set_var) {
        os << *itr;
        itr++;
        if (itr != set_var.end()) DUMPOUT << ", ";
        itr--;
    }
    DUMPOUT << "}";
    return os;
}
void print() {cout << endl;}
template <class Head, class... Tail>
void print(Head&& head, Tail&&... tail) {
  cout << head;
  if (sizeof...(tail) != 0) cout << " ";
  print(forward<Tail>(tail)...);
}
void dump_func() {DUMPOUT << '#'<<endl;}
template <typename Head, typename... Tail>
void dump_func(Head &&head, Tail &&... tail) {
    DUMPOUT << head;
    if (sizeof...(Tail) > 0) DUMPOUT << ", ";
    dump_func(std::move(tail)...);
}
#ifdef DEBUG_
#define DEB
#define dump(...)                                                              \
    DUMPOUT << "  " << string(#__VA_ARGS__) << ": "                            \
            << "[" << to_string(__LINE__) << ":" << __FUNCTION__ << "]"        \
            << endl                                                            \
            << "    ",                                                         \
        dump_func(__VA_ARGS__)
#else
#define DEB if (false)
#define dump(...)
#endif
template< typename T >
struct ruiseki{
    vector <T> s;
    int sz;
    ruiseki(vector <T> &a){
        sz = a.size()+1;
        s.resize(sz);
        for(int i=1;i<sz;i++) s[i] = s[i-1] + a[i-1];
    }
    T get(int a, int b){
        if(a < 0) a = 0; if(a > sz-1) a = sz-1;
        if(b > sz-1) b = sz-1;
        return s[b] - s[a];
    }
};
struct SegTree{
    int n;
    vector <pair <int,int> > node;
    SegTree(vector <int> v){
        int sz = v.size();
        n = 1;
        while(n<sz) n*=2;
        node.resize(2*n-1, {-INF,-INF});
        for(int i=0; i<sz; i++) node[i+n-1] = {v[i],i}; //一番下側
        for(int i=n-2;i>=0;i--) node[i] = max(node[2*i+1], node[2*i+2]);
    }
    int getnum(int p){
        return node[n-1+p].F;
    }
    int geti(int p){
        return node[n-1+p].S;
    }
    void update(int number, int val){
        number += n-1;
        node[number].F= val;
        while(number>0){
            number=(number-1)/2;
            node[number] = max(node[number*2+1], node[number*2+2]);
        }
    }
    pair <int,int> getmax(int a, int b, int k=0, int l = 0, int r = -1){ //0~n
        
        if(r<0) r=n;
        if(r<=a||b<=l) return {-INF,-INF};
        if(a <= l&& r<=b) return node[k];
        pair <int,int>  vl = getmax(a, b, 2*k+1, l, (l+r)/2);
        pair <int,int>  vr = getmax(a, b, 2*k+2, (l+r)/2, r);
        return max(vl, vr);
    }
    void showall(){
        for(int i=0;i<2*n-1;i++)cout<<node[i].F<<'&'<<node[i].S<<endl;
    }
};
signed main(void) { cin.tie(0); ios::sync_with_stdio(false);
    int n, c; cin>>n>>c;
    vector <int> x(n), rx, v(n), rv;
    for(int i=0;i<n;i++) cin>>x[i] >> v[i];
    rx = x;
    for(int i=0;i<n;i++) rx[i] = c - x[i];
    reverse(rx.begin(),rx.end());
    rv = v;
    reverse(rv.begin(),rv.end());
    dump(x, rx, v, rv);
    vector <int> resultR(n);
    vector <int> resultL(n);
    ruiseki<int> ruiv(v);
    ruiseki<int> ruirv(rv);
    for(int i=0;i<n;i++){
        resultR[i] = ruiv.get(0, i+1) - x[i];
        resultL[i] = ruirv.get(0, i+1) - rx[i];
    }
    SegTree segR(resultR), segL(resultL);
    int ans = max(0LL, max(segL.getmax(0, INF).F, segR.getmax(0, INF).F));
    for(int i=0;i<n;i++){
        int mal = segL.getmax(0, n-1-i).F;
        int tmp = resultR[i];
        chmax(ans, tmp + mal - x[i]);
    }
    rrep(i, n){
        int mar = segR.getmax(0, n-1-i).F;
        int tmp = resultL[i];
        chmax(ans, tmp + mar - rx[i]);
    }
    print(ans);
}


Measured memory_bytes: 18432