#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 <cstdbool>
#include <cstdint>
#include <ctgmath>
#include <cwchar>
#include <cwctype>
#endif

// C++
#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
using namespace std;

typedef long long ll;
//typedef pair<int,int> Pint;
typedef pair<ll, ll> P;
//typedef pair<int, pair<int, int>> P;
//typedef tuple<int,int,int> T;
typedef vector<ll> vec;
typedef vector<vec> mat;
#define rep(i, n) for(ll i = 0; i < n; i++)
#define revrep(i, n) for(ll i = n-1; i >= 0; i--)
ll max(ll a, ll b){return (a > b) ? a : b;}
ll min(ll a, ll b){return (a < b) ? a : b;}

ll INFL = 1000000000000000010;//10^18 = 2^60
int INF = 1000000000;//10^9
ll MOD  = 1000000007;
//vector<int> dy = {0,0,1,-1};
//vector<int> dx = {1,-1,0,0};
ll N, C;
vector<pair<ll, ll>> xv(100010);
vector<ll> toLeft(100010), toRight(100010);
//(i+1)まで食べれる時の最大値
vector<ll> fromLeft(100010), fromRight(100010);
vector<pair<ll, ll>> toLeftSum(100010);
vector<pair<ll, ll>> toRightSum(100010);

int main(){
  cin >> N >> C;
  xv.resize(N);
  rep(i, N) cin >> xv[i].first >> xv[i].second;
  sort(xv.begin(), xv.end());
  rep(i, N){
    toRightSum[i+1].first = xv[i].first;
    toRightSum[i+1].second = toRightSum[i].second + xv[i].second;
  }
  rep(i, N){
    toRight[i+1] = max(toRight[i], toRightSum[i+1].second - toRightSum[i+1].first);
  }
  rep(i, N){
    fromRight[i+1] = max(fromRight[i], toRightSum[i+1].second - 2 * toRightSum[i+1].first);
  }

  rep(i, N) xv[i].first = C - xv[i].first;
  sort(xv.begin(), xv.end());
  rep(i, N){
    toLeftSum[i+1].first = xv[i].first;
    toLeftSum[i+1].second = toLeftSum[i].second + xv[i].second;
  }
  rep(i, N){
    toLeft[i+1] = max(toLeft[i], toLeftSum[i+1].second - toLeftSum[i+1].first);
  }
  rep(i, N){
    fromLeft[i+1] = max(fromLeft[i], toLeftSum[i+1].second - 2 * toLeftSum[i+1].first);
  }
  ll ans = 0;
  for(int i = 0; i < N + 1; i++){
    ans = max(ans, toRight[i] + fromLeft[N - i]);
    ans = max(ans, toLeft[i] + fromRight[N - i]);
  }
  cout << ans << endl;

}


Measured memory_bytes: 8064