#include <algorithm>
#include <complex>
#include <cstdlib>
#include <ctime>
#include <time.h>
#include <iomanip>
#include <iostream>
#include <map>
#include <math.h>
#include <queue>
#include <random>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <vector>
#include <numeric>
#include <limits>
#include <type_traits>


using namespace std;
#define SAY_YES cout << "YES" << endl;
#define SAY_Yes cout << "Yes" << endl;
#define SAY_NO cout << "NO" << endl;
#define SAY_No cout << "No" << endl;
#define IFYES(TRUE_OR_FALSE)   \
    if (TRUE_OR_FALSE)         \
    {                          \
        cout << "YES" << endl; \
    }                          \
    else                       \
    {                          \
        cout << "NO" << endl;  \
    }
#define IFYes(TRUE_OR_FALSE)   \
    if (TRUE_OR_FALSE)         \
    {                          \
        cout << "Yes" << endl; \
    }                          \
    else                       \
    {                          \
        cout << "No" << endl;  \
    }
#define IFyes(TRUE_OR_FALSE)   \
    if (TRUE_OR_FALSE)         \
    {                          \
        cout << "yes" << endl; \
    }                          \
    else                       \
    {                          \
        cout << "no" << endl;  \
    }
#define DEBUG_OUTPUT_ARRAY(XXX, ONE)                            \
    for (int i = 0; i < (ONE); i++)                             \
    {                                                           \
        cout << "DEBUG: i = " << i << " -> " << XXX[i] << endl; \
    }
#define DEBUG_OUTPUT_ARRAY2(XXX, ONE, TWO)                             \
    for (int i = 0; i < (ONE); i++)                                    \
    {                                                                  \
        cout << "<<< i = " << i << " >>>" << endl;                     \
        for (int j = 0; j < (TWO); j++)                                \
        {                                                              \
            cout << "DEBUG: j = " << j << " -> " << XXX[i][j] << endl; \
        }                                                              \
    }
#define DEBUG_OUTPUT_ARRAY2_BOX(XXX, ONE, TWO) \
    for (int i = 0; i < (ONE); i++)            \
    {                                          \
        cout << i << "  ";                     \
        for (int j = 0; j < (TWO); j++)        \
        {                                      \
            cout << XXX[i][j] << " ";          \
        }                                      \
        cout << endl;                          \
    }

typedef pair<long long int, long long int> pll;


const long long int mod = 1000000007;
const long long int INF = 1e18;
const long double PI=3.14159265358979323; 

long long int mi=0;
long long int N,M,C,x[100020],v[100200]={},res=0,abssum=0;

long long int rig[200005]={};
long long int le[200005]={};
string S;

pll clo[200020],revclo[200020];
int main()
{
    cout << fixed << setprecision(18);
    cin>>N>>C;
    for(long long int i = 1; i <= N; i++){
        cin>>x[i]>>v[i];
    }
    clo[0]={0,0};
    revclo[0]={0,0};
    for(long long int i = 1; i <= N; i++){
        clo[i]={clo[i-1].first+v[i],x[i]};
        revclo[i]={revclo[i-1].first+v[N-i+1],C-x[N-i+1]};

    }
    //全部食べる場合
    for(long long int i = 0; i <= N; i++){
        
        long long int tmp=clo[i].first+revclo[N-i].first;
        res=max({res,tmp-(2*clo[i].second+revclo[N-i].second),tmp-(clo[i].second+2*revclo[N-i].second)});
    }
    //右行った時の最大
    for(long long int i = 1; i <= N; i++){
        rig[i]=max(rig[i-1],clo[i].first-clo[i].second);
        le[i]=max(le[i-1],revclo[i].first-revclo[i].second);
    }
    //左からi個食べる場合(l->r)
    for(long long int i = 0; i <= N; i++){
        long long int tmp=revclo[i].first-2*revclo[i].second;
        res=max(res,tmp+rig[N-i]);
        tmp=clo[i].first-2*clo[i].second;
        res=max(res,tmp+le[N-i]);
    }
    cout<<res<<endl;
}    


Measured memory_bytes: 10752