//include
//------------------------------------------
#include <vector>
#include <list>
#include <map>
#include <unordered_map>
#include <climits>
#include <set>
#include <unordered_set>
#include <deque>
#include <stack>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>
#include <sstream>
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <ctime>
#include <queue>
#include <random>
#include <chrono>
#include <complex>
#include <regex>
#include <locale>
#include <random>
#include <type_traits>

using namespace std;

// typedef
//------------------------------------------
typedef long long LL;
typedef vector<int> VI;
typedef vector<bool> VB;
typedef vector<char> VC;
typedef vector<double> VD;
typedef vector<string> VS;
typedef vector<LL> VLL;
typedef vector<VI> VVI;
typedef vector<VB> VVB;
typedef vector<VS> VVS;
typedef vector<VLL> VVLL;
typedef vector<VVI> VVVI;
typedef vector<VVLL> VVVLL;
typedef pair<int, int> PII;
typedef pair<LL, LL> PLL;
typedef pair<int, string> PIS;
typedef pair<string, int> PSI;
typedef pair<string, string> PSS;
typedef vector<PII> VPII;
typedef vector<PLL> VPLL;
typedef vector<VPII> VVPII;
typedef vector<VPLL> VVPLL;
typedef vector<VS> VVS;
typedef map<int, int> MII;
typedef map<LL, LL> MLL;
typedef map<string, int> MSI;
typedef map<int, string> MIS;

// to-method
//------------------------------------------
inline int toInt(string s) {
    int v;
    istringstream sin(s);
    sin >> v;
    return v;
}

inline LL toLongLong(string s) {
    LL v;
    istringstream sin(s);
    sin >> v;
    return v;
}

template<class T>
inline string toString(T x) {
    ostringstream sout;
    sout << x;
    return sout.str();
}

inline VC toVC(string s) {
    VC data(s.begin(), s.end());
    return data;
}



// container util
//------------------------------------------
#define ALL(a)  (a).begin(),(a).end()
#define SZ(a) int((a).size())
#define EACH(i, arr) for(typeof((arr).begin()) i=(arr).begin(); i!=(arr).end(); ++i)
#define EXIST(str, e) ((str).find(e)!=(str).end())
#define COUNT(arr, v) count((arr).begin(), (arr).end(), v)
#define SEARCH(v, w) search((v).begin(), (v).end(), (w).begin(), (w).end())
#define SORT(c) sort((c).begin(),(c).end())
#define RSORT(c) sort((c).rbegin(),(c).rend())
#define REVERSE(c) reverse((c).begin(), (c).end())
#define ROTATE_LEFT(arr, c) rotate((arr).begin(), (arr).begin()+(c), (arr).end())
#define ROTATE_RIGHT(arr, c) rotate((arr).rbegin(), (arr).rbegin() + (c), (arr).rend())
#define SUMI(arr) accumulate((arr).begin(), (arr).end(), 0)
#define SUMD(arr) accumulate((arr).begin(), (arr).end(), 0.)
#define SUMLL(arr) accumulate((arr).begin(), (arr).end(), 0LL)
#define SUMS(arr) accumulate((arr).begin(), (arr).end(), string())
#define MULD(arr) accumulate((arr).begin(), (arr).end(), 1., multiplies<double>())
#define MULLL(arr) accumulate((arr).begin(), (arr).end(), 1LL, multiplies<long long>())
#define UB(arr, n) upper_bound((arr).begin(), (arr).end(), n)
#define LB(arr, n) lower_bound((arr).begin(), (arr).end(), n)
#define OF_ALL(arr, func) all_of((arr).begin(), (arr).end(), (func))
#define OF_NONE(arr, func) none_of((arr).begin(), (arr).end(), (func))
#define OF_ANY(arr, func) any_of((arr).begin(), (arr).end(), (func))
#define PB push_back
#define MP make_pair
#define ft first
#define sd second



// input output
//------------------------------------------
#define GL(s) getline(cin, (s))
#define INIT() std::ios::sync_with_stdio(false);std::cin.tie(0)
#define OUT(d) std::cout<<(d)
#define OUT_L(d) std::cout<<(d)<<endl
#define FOUT(n, data) std::cout<<std::fixed<<std::setprecision(n)<<(data)
#define FOUT_L(n, data) std::cout<<std::fixed<<std::setprecision(n)<<(data)<<"\n"
#define EL() printf("\n")
#define SHOW_VECTOR(v) {std::cerr << #v << "\t:";for(const auto& xxx : v){std::cerr << xxx << " ";}std::cerr << "\n";}
#define SHOW_MAP(v) {std::cerr << #v << endl; for(const auto& xxx: v){std::cerr << xxx.first << " " << xxx.second << "\n";}}
#define Yes() printf("Yes\n")
#define No() printf("No\n")
#define YES() printf("YES\n")
#define NO() printf("NO\n")
#define Yay() printf("Yay!\n")
#define Nnn() printf(":(\n")


template<typename T1, typename T2>
istream &operator>>(istream &in, pair<T1, T2> &p) {
    in >> p.first >> p.second;
    return in;
}

template<typename T>
istream &operator>>(istream &in, vector<T> &v) {
    for (auto &x: v)
        in >> x;
    return in;
}


template<typename T1, typename T2>
ostream &operator<<(ostream &out, const std::pair<T1, T2> &p) {
    out << "[" << p.first << ", " << p.second << "]" << "\n";
    return out;
}


//repetition
//------------------------------------------
#define FOR(i, a, b) for(int i=(a);i<(b);++i)
#define RFOR(i, a, b) for(int i=(b)-1;i>=(a);--i)
#define REP(i, n)  FOR(i,0,n)
#define RREP(i, n) for(int i = n-1;i >= 0;i--)
#define FORLL(i, a, b) for(LL i=LL(a);i<LL(b);++i)
#define RFORLL(i, a, b) for(LL i=LL(b)-1;i>=LL(a);--i)
#define REPLL(i, n) for(LL i=0;i<LL(n);++i)
#define RREPLL(i, n) for(LL i=LL(n)-1;i>=0;--i)
#define FOREACH(x, arr) for(auto &(x) : (arr))
#define FORITER(x, arr) for(auto (x) = (arr).begin(); (x) != (arr).end(); ++(x))

template<class T>
void chmin(T &a, const T b) { a = min(a, b); }
template<class T>
void chmax(T &a, const T b) { a = max(a, b); }

//------------------------------------------
//------------------------------------------

constexpr LL INF = 1e16;

template<typename T>
class CulSum {
private:
    long long N;
    vector<T> a;
    vector<T> aa;
    vector<T> b;
    vector<T> bb;

    function<T(T, T)> func;

public:

    /// @param a original array
    CulSum(vector<T> _a) {
        N = _a.size();
        a = _a;
        b = _a;
        reverse(b.begin(), b.end());
        this->aa = vector<T>(N + 1, T{});
        this->bb = vector<T>(N + 1, T{});
        for (int i = 0; i < N; i++) aa[i + 1] = a[i] + aa[i];
        for (int i = 0; i < N; i++) bb[i + 1] = b[i] + bb[i];
    }

    /// @param a original array
    CulSum(vector<T> _a, function<T(T, T)> func) : func(func) {
        N = _a.size();
        a = _a;
        b = _a;
        reverse(b.begin(), b.end());
        this->aa = vector<T>(N + 1, T{});
        this->bb = vector<T>(N + 1, T{});
        for (int i = 0; i < N; i++) aa[i + 1] = func(a[i], aa[i]);
        for (int i = 0; i < N; i++) bb[i + 1] = func(b[i], bb[i]);
    }

    /// @brief 0-index original
    /// @param l original close
    /// @param r original open
    T get_sum(int l, int r) {
        return aa[r] - aa[l];
    }

    /// @brief 0-index original
    /// @param l original close
    /// @param r original open
    T get_sum_reverse(int l, int r) {
        l = N - l;
        r = N - r;
        return bb[l] - bb[r];
    }

    /// @brief 0-index original
    /// @param k length
    T get_sum_lead_k_len(int k) {
        return get_sum(0, k);
    }

    /// @brief 0-index original
    /// @param k length
    T get_sum_tail_k_len(int k) {
        return get_sum_reverse(N - k, N);
    }

    void show() {
        for (int i = 0; i < N + 1; i++) cout << aa[i] << " ";
        cout << endl;
    }

    void show_reverse() {
        for (int i = N; i >= 0; i--) cout << bb[i] << " ";
        cout << endl;
    }

};

template<typename T>
struct SegmentTree {
private:
    using Func = function<T(T, T)>;

    int n;
    vector<T> node;

    T init_v;
    Func func;

public:

    SegmentTree(vector<T> a, Func _func, T _init_v) {
        int sz = a.size();
        n = 1;
        init_v = _init_v;
        func = _func;

        while (n < sz) n *= 2;
        node.resize(2 * n, init_v);
        for (int i = 0; i < sz; i++) node[i + n - 1] = a[i];
        for (int i = n - 2; i >= 0; i--) node[i] = func(node[i * 2 + 1], node[i * 2 + 2]);
    }

    void update(int pos, T v) {
        int k = pos + n - 1;
        node[k] = v;
        while (k > 0) {
            k = (k - 1) / 2;
            node[k] = func(node[k * 2 + 1], node[k * 2 + 2]);
        }
    }

    T get(int pos) {
        int k = pos + n - 1;
        return node[k];
    }

    T query(int a, int b, int k = 0, int l = 0, int r = -1) {
        if (r < 0) r = n;
        if (r <= a || b <= l) return init_v;
        if (a <= l && r <= b)return node[k];
        T lv = query(a, b, k * 2 + 1, l, (l + r) / 2);
        T rv = query(a, b, k * 2 + 2, (l + r) / 2, r);
        return func(lv, rv);
    }

};

void half_path(LL N, vector<LL> &x, vector<LL> &v, LL &ans) {
    CulSum<LL> culSum(v);
    for (int i = 0; i < N; i++) {
        LL d = culSum.get_sum(0, i + 1) - x[i];
        ans = max(ans, d);
    }
}

void half_half_half(LL N, vector<LL> &x, vector<LL> &v, vector<LL> &rx, vector<LL> &rv, LL &ans) {
    CulSum<LL> culSum(v);
    CulSum<LL> r_culSum(rv);
    vector<LL> other_score(N);
    for (int i = 0; i < N; i++) {
        LL d = r_culSum.get_sum(0, i + 1) - rx[i];
        other_score[i] = d;
    }

    SegmentTree<LL> segmentTree(other_score, [](const LL a, const LL b) { return max(a, b); }, -INF);

    for (int i = 0; i < N; i++) {
        LL d = culSum.get_sum(0, i + 1) - 2 * x[i];
        d += segmentTree.query(0, N - i - 1);
        ans = max(ans, d);
    }
}

int main() {

    LL N, C;
    cin >> N >> C;

    vector<LL> x(N), v(N);
    for (int i = 0; i < N; i++) cin >> x[i] >> v[i];

    vector<LL> rx = x, rv = v;
    REVERSE(rx);
    for (int i = 0; i < N; i++) rx[i] = C - rx[i];
    REVERSE(rv);

    LL ans = 0;
    half_path(N, x, v, ans);
    half_path(N, rx, rv, ans);

    half_half_half(N, x, v, rx, rv, ans);
    half_half_half(N, rx, rv, x, v, ans);

    cout << ans << endl;

    return 0;
}






































Measured memory_bytes: 13340