#include <bits/stdc++.h>
#define whlie while
#define pb push_back
#define eb emplace_back
#define fi first
#define se second
#define rep(i,N) for(int i = 0; i < (N); i++)
#define repr(i,N) for(int i = (N) - 1; i >= 0; i--)
#define rep1(i,N) for(int i = 1; i <= (N) ; i++)
#define repr1(i,N) for(int i = (N) ; i > 0 ; i--)
#define each(x,v) for(auto& x : v)
#define all(v) (v).begin(),(v).end()
#define sz(v) ((int)(v).size())
#define ini(...) int __VA_ARGS__; in(__VA_ARGS__)
#define inl(...) ll __VA_ARGS__; in(__VA_ARGS__)
#define ins(...) string __VA_ARGS__; in(__VA_ARGS__)
using namespace std; void solve();
using ll = long long; using vl = vector<ll>;
using vi = vector<int>; using vvi = vector< vector<int> >;
constexpr int inf = 1001001001;
constexpr ll infLL = (1LL << 61) - 1;
struct IoSetupNya {IoSetupNya() { cin.tie(nullptr); ios::sync_with_stdio(false); cout << fixed << setprecision(15); cerr << fixed << setprecision(7);} } iosetupnya;
template<typename T, typename U> inline bool amin(T &x, U y) { return (y < x) ? (x = y, true) : false; }
template<typename T, typename U> inline bool amax(T &x, U y) { return (x < y) ? (x = y, true) : false; }
template<typename T, typename U> ostream& operator <<(ostream& os, const pair<T, U> &p) { os << p.first << " " << p.second; return os; }
template<typename T, typename U> istream& operator >>(istream& is, pair<T, U> &p) { is >> p.first >> p.second; return is; }
template<typename T> ostream& operator <<(ostream& os, const vector<T> &v) { int s = (int)v.size(); rep(i,s) os << (i ? " " : "") << v[i]; return os; }
template<typename T> istream& operator >>(istream& is, vector<T> &v) { for(auto &x : v) is >> x; return is; }
void in(){} template <typename T,class... U> void in(T &t,U &...u){ cin >> t; in(u...);}
void out(){cout << "\n";} template <typename T,class... U> void out(const T &t,const U &...u){ cout << t; if(sizeof...(u)) cout << " "; out(u...);}
template<typename T>void die(T x){out(x); exit(0);}
#ifdef NyaanDebug
  #include "NyaanDebug.h"
  #define trc(...) do { cerr << #__VA_ARGS__ << " = "; dbg_out(__VA_ARGS__);} while(0)
  #define trca(v,N) do { cerr << #v << " = "; array_out(v , N);cout << endl;} while(0)
#else
  #define trc(...)
  #define trca(...)
  int main(){solve();}
#endif
using P = pair<ll,ll>; using vp = vector<P>;
constexpr int MOD = /** 1000000007; //*/ 998244353;
/////////

// セグ木
template<typename T,typename F>
struct ST{

  int size;
  vector<T> seg;
  const F func;
  const T UNIT;
  
  ST(int N,F func , T UNIT): func(func) , UNIT(UNIT) {
    size = 1;
    while(size < N) size <<= 1;
    seg.assign(2 * size, UNIT);
  }

  ST(const vector<T> &v,F func , T UNIT) : func(func) , UNIT(UNIT){
    // initialize
    int N = (int)v.size();
    size = 1;
    while(size < N) size <<= 1;
    seg.assign(2 * size , UNIT);
    // set
    for(int i = 0; i < N; i++){
      seg[i + size] = v[i];
    }
    build();
  }

  void set(int k, T x){
    seg[k + size] = x;
  }

  void build(){
    for(int k = size-1; k > 0; k--){
      seg[k] = func(seg[2 * k] , seg[2 * k + 1] );
    }
  }
  
  void update(int k, T x){
    k += size; seg[k] = x;
    while(k >>= 1){
      seg[k] = func( seg[2 * k] , seg[2 * k + 1] );
    }
  }

  void add(int k , T x){
    k += size; seg[k] += x;
    while(k >>= 1){
      seg[k] = func(seg[2 * k] , seg[2 * k + 1] );
    }
  }
  
  // 半開区間[a,b)へのクエリ
  T query(int a, int b){
    T L = UNIT, R = UNIT;
    for(a+=size,b+=size; a<b; a>>=1,b>>=1){
      if(a & 1) L = func(L,seg[a++]);
      if(b & 1) R = func(seg[--b],R);
    }
    return func(L, R);
  }

  T operator[](const int &k) const{
    return seg[k + size];
  }

};

void solve(){
  ini(N); inl(C);
  vp v(N); in(v);

  auto f=[](ll a,ll b){return max(a , b);};

  vl clockdat(N) , countdat(N);
  // 時計回り
 for(ll i = 0 , cur = 0 ; i < N ; i++){
   cur += v[i].se;
   // 
   clockdat[i] = cur - v[i].fi;
 }

 for(ll i = N - 1 , cur = 0 ; i >= 0 ; i--){
   cur += v[i].se;
   countdat[i] = cur - (C - v[i].fi);
 }
 
  ST<ll,decltype(f)> cl(clockdat,f,-infLL) , co(countdat,f,-infLL);

  ll ans = 0;
  // 初手時計回りにiまで食べる
  rep(i , N){
    // 食べる
    ll cur = cl[i];
    // 原点に戻って逆回りをする
    ll nya = max(0LL , -v[i].fi + co.query(i + 1 , N));
    amax(ans , cur + nya);
  }

  // 初手反時計
  rep(i , N){
    // i ~ N + 1を食べる
    ll cur = co[i];
    // 原点に戻り逆回り
    ll nya = max(0LL , -(C - v[i].fi) + cl.query(0 , i));
    amax(ans , cur + nya);
  }

  out(ans);

}

Measured memory_bytes: 7552