#include<iostream>
#include<string>
#include<algorithm>
#include<cstdlib>
#include<cmath>
#include<vector>
#include<list>
#include<cstdint>
#include<bits/stdc++.h>
#include<queue>
#include<set>

#define rep(i, n) for(int i = 0; i < (int)(n); i++)
#define rep_r(i, n) for(int i = n-1; i >= 0; i--)
#define rep1(i, n) for(int i = 1; i <= (int)(n); i++)
#define REP(i,n) for(int i=0, i##_len=(n); i<i##_len; ++i)
#define all(x) (x).begin(),(x).end()
#define SZ(x) ((ll)(x).size())
#define bit(n) (1LL<<(n))
#define UNIQUE(v) v.erase( unique(v.begin(), v.end()), v.end() );

#define INF bit(60)
#define pb push_back
#define mod 1000000007

template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return 1; } return 0; }

using namespace std;
using uif=uint_fast64_t;
using ll=long long int;

int qp(int a,ll b){int ans=1;do{if(b&1)ans=1ll*ans*a%mod;a=1ll*a*a%mod;}while(b>>=1);return ans;}
int qp(int a,ll b,int mo){int ans=1;do{if(b&1)ans=1ll*ans*a%mo;a=1ll*a*a%mo;}while(b>>=1);return ans;}

int gcd(int a,int b){return b?gcd(b,a%b):a;}
int dx[4]={1,0,-1,0};
int dy[4]={0,1,0,-1};


#define FACSIZE 2002

ll invfac[FACSIZE];
ll fac[FACSIZE];

ll mod_pow(ll ,ll );
ll mod_add(ll ,ll );
ll mod_mul(ll ,ll );
ll mod_div(ll ,ll );
void make_fact(ll );
void make_invfact(ll );



int main(void){

  ll n,c;
  cin>>n>>c;

  vector<ll> x(n);
  vector<ll> v(n);
  rep(i,n) cin>>x[i]>>v[i];

  ll ans=0;

  vector< pair<ll,ll> > total(n+1);
  vector< pair<ll,ll> > total_d(n+1);
  vector< ll > total_max(n+1);
  vector< ll > total_d_max(n+1);

  total[0] = make_pair(0,0);
  total_d[n] = make_pair(0,0);
  total_max[0]=0;
  total_d_max[n] = 0;
  
  /*
  c/=10000000;
  rep(i,n) x[i]/=10000000,v[i]/=10000000;
  */

  for(ll i=1;i<=n;i++){
    total[i]=make_pair( total[i-1].first+v[i-1] , x[i-1] );
    total_max[i] = max(total_max[i-1],total[i].first - total[i].second);
  }
  for(ll i=n-1;i>=0;i--){
    total_d[i]=make_pair( total_d[i+1].first+v[i] , c-x[i] );
    total_d_max[i] = max( total_d_max[i+1], total_d[i].first - total_d[i].second );
  }
  /*
  for(auto p:total) cout << p.first << "," << p.second << ":" << p.first - p.second << " :: " << p.first - 2*p.second << endl;
  cout << endl;
  for(auto p:total_d) cout << p.first << "," << p.second << ":" << p.first - p.second << " :: "  << p.first - 2*p.second << endl;
  cout << endl;
  for(auto x:total_max) cout << x << endl;
  cout << endl;
  for(auto x:total_d_max) cout << x << endl;
  cout << endl;
  */

  for(ll i=0;i<=n;i++){
    ans=max(ans,total[i].first-total[i].second);
  }
  for(ll i=n;i>=0;i--){
    ans=max(ans,total[i].first-total[i].second);
  }

  for(ll i=0;i<=n;i++)  {
    ans = max(ans, total[i].first - 2*total[i].second + total_d_max[i] );
  }
  for(ll i=n;i>=0;i--){
    ans = max(ans, total_d[i].first - 2*total_d[i].second + total_max[i] );
  }

  cout << ans << endl;

  return 0;

}

ll mod_pow(ll x,ll r){
  if(r==0) return 1;
  else if(r==1) return x%mod;
  else if(r%2==0){
    ll t=mod_pow(x,r/2)%mod;
    return t*t %mod;
  } 
  else{
    ll t=mod_pow(x,r/2)%mod;
    ll k= t*t %mod;
    return k*x%mod;
  } 
}
ll mod_add(ll a,ll b){
  return (a+b)%mod;
}

ll mod_mul(ll a,ll b){
  return (a*b)%mod;
}

ll mod_div(ll a,ll b){
  return mod_mul(a,mod_pow(b,mod-2));
}

void make_fact(ll n){
  fac[0]=1;
  rep(i,n){
    fac[i+1]=mod_mul(fac[i],i+1);
  }  
}

void make_invfact(ll n){
  invfac[n]= mod_pow( fac[n] ,mod-2);
  for(int i=n-1;i>=0;i--){
    invfac[i]=mod_mul(invfac[i+1],i+1);
  }

}


Measured memory_bytes: 6528