#ifndef _GLIBCXX_NO_ASSERT
#include <cassert>
#endif
#include <cctype>
#include <cerrno>
#include <cfloat>
#include <ciso646>
#include <climits>
#include <clocale>
#include <cmath>
#include <csetjmp>
#include <csignal>
#include <cstdarg>
#include <cstddef>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>
#if __cplusplus >= 201103L
#include <ccomplex>
#include <cfenv>
#include <cinttypes>
#include <cstdalign>
#include <cstdbool>
#include <cstdint>
#include <ctgmath>
#include <cwchar>
#include <cwctype>
#endif
#include <algorithm>
#include <bitset>
#include <complex>
#include <deque>
#include <exception>
#include <fstream>
#include <functional>
#include <iomanip>
#include <ios>
#include <iosfwd>
#include <iostream>
#include <istream>
#include <iterator>
#include <limits>
#include <list>
#include <locale>
#include <map>
#include <memory>
#include <new>
#include <numeric>
#include <ostream>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <stdexcept>
#include <streambuf>
#include <string>
#include <typeinfo>
#include <utility>
#include <valarray>
#include <vector>
#if __cplusplus >= 201103L
#include <array>
#include <atomic>
#include <chrono>
#include <condition_variable>
#include <forward_list>
#include <future>
#include <initializer_list>
#include <mutex>
#include <random>
#include <ratio>
#include <regex>
#include <scoped_allocator>
#include <system_error>
#include <thread>
#include <tuple>
#include <typeindex>
#include <type_traits>
#include <unordered_map>
#include <unordered_set>
#endif
#define y0 qvya13579
#define y1 qvyb24680
#define j0 qvja13579
#define j1 qvjb24680
#define next qvne13579xt
#define prev qvpr13579ev
#define INF 1000000007
#define PI acos(-1.0)
#define endl "\n"
#define IOS cin.tie(0);ios::sync_with_stdio(false)
#define M_P make_pair
#define PU_B push_back
#define PU_F push_front
#define PO_B pop_back
#define PO_F pop_front
#define U_B upper_bound
#define L_B lower_bound
#define B_S binary_search
#define PR_Q priority_queue
#define FIR first
#define SEC second
#if __cplusplus < 201103L
#define stoi(argument_string) atoi((argument_string).c_str())
#endif
#define REP(i,n) for(int i=0;i<(int)(n);++i)
#define REP_R(i,n) for(int i=((int)(n)-1);i>=0;--i)
#define FOR(i,m,n) for(int i=((int)(m));i<(int)(n);++i)
#define FOR_R(i,m,n) for(int i=((int)(m)-1);i>=(int)(n);--i)
#define ALL(v) (v).begin(),(v).end()
#define RALL(v) (v).rbegin(),(v).rend()
#define SIZ(x) ((int)(x).size())
#define COUT(x) cout<<(x)<<endl
#define CIN(x) cin>>(x)
#define CIN2(x,y) cin>>(x)>>(y)
#define CIN3(x,y,z) cin>>(x)>>(y)>>(z)
#define CIN4(x,y,z,w) cin>>(x)>>(y)>>(z)>>(w)
#define SCAND(x) scanf("%d",&(x))
#define SCAND2(x,y) scanf("%d%d",&(x),&(y))
#define SCAND3(x,y,z) scanf("%d%d%d",&(x),&(y),&(z))
#define SCAND4(x,y,z,w) scanf("%d%d%d%d",&(x),&(y),&(z),&(w))
#define SCANLLD(x) scanf("%lld",&(x))
#define SCANLLD2(x,y) scanf("%lld%lld",&(x),&(y))
#define SCANLLD3(x,y,z) scanf("%lld%lld%lld",&(x),&(y),&(z))
#define SCANLLD4(x,y,z,w) scanf("%lld%lld%lld%lld",&(x),&(y),&(z),&(w))
#define PRINTD(x) printf("%d\n",(x))
#define PRINTLLD(x) printf("%lld\n",(x))
typedef long long int lli;
using namespace std;
 
int ctoi(char c)
{
  if( c >= '0' and c <= '9' )
    {
      return (int)(c-'0');
    }
 
  return -1;
}
 
int alphabet_pos(char c) 
{
  if( c >= 'a' and c <= 'z' )
    {
      return (int)(c-'a');
    }
 
  return -1;
}
 
 
int alphabet_pos_capital(char c)
{
  if( c >= 'A' and c <= 'Z' )
    {
      return (int)(c-'A');
    }
 
  return -1;
}
 
 
vector<string> split(string str, char ch)
{
  int first = 0;
  int last = str.find_first_of(ch);
  
  if(last == string::npos)
    {
      last = SIZ(str);
    }
 
  vector<string> result;
 
  while( first < SIZ(str) )
    {
      string Ssubstr(str, first, last - first);
      result.push_back(Ssubstr);
      first = last + 1;
      last = str.find_first_of(ch, first);
 
      if(last == string::npos)
	{
	  last = SIZ(str);
	}
    }
  
  return result;
}
 
/*------------------ the end of the template -----------------------*/
 
 
lli lmax( lli a, lli b )
{
  if( a < b )
    {
      return b;
      
    }
  else
    {
      return a;
      
    }
}
 
lli lmin( lli a, lli b )
{
  if( a > b )
    {
      return b;
      
    }
  else
    {
      return a;
      
    }
}
 
int main()
{
  //IOS; /* making cin faster */
  int N;
  SCAND(N);
  lli C;
  SCANLLD(C);
 
  vector<lli> x(N+2);
  vector<int> v(N);
  vector<lli> cuv(N+1);
 
  x[0] = 0;
  x[N+1] = C;
  cuv[0] = 0;
  
 
  REP(i,N)
    {
      SCANLLD(x[i+1]);
      SCAND(v[i]);
      cuv[i+1] = cuv[i] + v[i];
      
    }
 
  vector<pair<lli,int> > mx_cw(N,make_pair(0,0));
  vector<pair<lli,int> > mx_ccw(N,make_pair(0,0));
 
  int id = 0;
  
  REP(i,N)
    {
      lli temp = cuv[i+1] - x[i+1];
      
      if( i == 0 )
	{
	  mx_cw[i].first = lmax(0,temp);
 
	  if( temp > 0)
	    {
	      mx_cw[i].second = i+1;
	      
	    }
	  else
	    {
	      mx_cw[i].second = 0;
	    }
	}
      else
	{
 
	  if( temp > mx_cw[i-1].first )
	    {
	      mx_cw[i].second = i+1;
	      
	    }
	  else
	    {
	      mx_cw[i].second = mx_cw[i-1].second;
	    }
 
	  mx_cw[i].first = lmax(temp,mx_cw[i-1].first);
	  
	}
    }
  
  REP_R(i,N)
    {
      
      lli temp =  cuv[N] - cuv[i]  - ( C - x[i+1] );
 
      if( i == N-1  )
	{
	  mx_ccw[i].first = lmax(0,temp);
	  
	  if( temp > 0)
	    {
	      mx_ccw[i].second = i+1;
	      
	    }
	}
      else
	{
 
	  if( temp > mx_ccw[i+1].first )
	    {
	      mx_ccw[i].second = i+1;
	      
	    }
	  else
	    {
	      mx_ccw[i].second = mx_ccw[i+1].second;
	    }
	  
	  mx_ccw[i].first  = lmax(temp,mx_ccw[i+1].first);
	}
      
    }
 
  lli mx = lmax(mx_cw[N-1].first , mx_ccw[0].first);
  
  REP(i,N-1)
    {
      if( mx < mx_cw[i].first + mx_ccw[i+1].first - lmin( x[mx_cw[i].second] , C - x[mx_ccw[i+1].second]) )
	{
	  mx = mx_cw[i].first + mx_ccw[i+1].first - lmin( x[mx_cw[i].second] , C - x[mx_ccw[i+1].second]);
	  
	  
	}
    }
 
  PRINTLLD(mx);
  
  
 
}


Measured memory_bytes: 5376