#include <bits/stdc++.h>

using namespace std;

#define int long long
#define FOR(i, a, b) for(ll i = (a); i < (b); ++i)
#define FORR(i, a, b) for(ll i = (a); i > (b); --i)
#define REP(i, n) for(ll i = 0; i < (n); ++i)
#define REPR(i, n) for(ll i = n; i >= 0; i--)
#define FOREACH(x, a) for(auto &(x) : (a))
#define VECCIN(x)                                                              \
    for(auto &youso_ : (x)) cin >> youso_
#define bitcnt __builtin_popcount
#define SZ(x) ((ll)(x).size())
#define fi first
#define se second
#define All(a) (a).begin(), (a).end()
template <typename T = long long> inline T IN() {
    T x;
    cin >> x;
    return (x);
}
inline void CIN() {}
template <class Head, class... Tail>
inline void CIN(Head &&head, Tail &&... tail) {
    cin >> head;
    CIN(move(tail)...);
}
#define CINT(...)                                                              \
    int __VA_ARGS__;                                                           \
    CIN(__VA_ARGS__)
#define LCIN(...)                                                              \
    ll __VA_ARGS__;                                                            \
    CIN(__VA_ARGS__)
#define SCIN(...)                                                              \
    string __VA_ARGS__;                                                        \
    CIN(__VA_ARGS__)
#define Yes(a) cout << (a ? "Yes" : "No") << "\n"
#define YES(a) cout << (a ? "YES" : "NO") << "\n"
#define Printv(v)                                                              \
    {                                                                          \
        FOREACH(x, v) { cout << x << " "; }                                    \
        cout << "\n";                                                          \
    }
template <typename T = string> inline void eputs(T s) {
    cout << s << "\n";
    exit(0);
}
template <typename A, size_t N, typename T>
void Fill(A (&array)[N], const T &val) {
    std::fill((T *)array, (T *)(array + N), val);
}
template <typename T> using PQG = priority_queue<T, vector<T>, greater<T>>;
template <typename T> using PQ = priority_queue<T>;

typedef long long ll;
typedef unsigned long long ul;
typedef vector<ll> VL;
typedef pair<ll, ll> PL;

const int INF = 1e9;
const int MOD = 1e9 + 7;
// const int MOD = 998244353;
const ll LINF = 1e18 + 1;
const ll dx[] = {1, -1, 0, 0};
const ll dy[] = {0, 0, 1, -1};

ll N, C;
vector<PL> sushi;
VL sum, rsum;
VL opt, ropt;

signed main() {
    cin >> N >> C;
    REP(i, N) {
        LCIN(x, v);
        sushi.emplace_back(x, v);
    }
    sum.resize(N + 2);
    rsum.resize(N + 2);
    opt.resize(N + 2);
    ropt.resize(N + 2);
    REP(i, N) {
        sum[i + 1] = sum[i] + (sushi[i].se - sushi[i].fi);
        rsum[N - i] =
            rsum[N + 1 - i] + sushi[N - 1 - i].se - (C - sushi[N - 1 - i].fi);
        if(i > 0) {
            sum[i + 1] += sushi[i - 1].fi;
            rsum[N - i] += C - sushi[N - i].fi;
        }
    }
    REP(i, N + 1) {
        opt[i + 1] = max(opt[i], sum[i + 1]);
        ropt[N - i] = max(ropt[N + 1 - i], rsum[N - i]);
    }

    ll ans = 0;
    // clock-counterclock
    REP(i, N + 1) {
        if(i == 0)
            ans = max(ans, ropt[0]);
        else
            ans = max({ans, sum[i] - sushi[i - 1].fi + ropt[i + 1], sum[i]});
    }

    // counterclock-clock
    REP(i, N + 1) {
        if(i == 0)
            ans = max(ans, opt[N + 1]);
        else
            ans = max({ans, rsum[N + 1 - i] - (C - sushi[N - i].fi) + opt[N - i], rsum[N + 1 - i]});
    }
    cout << ans << "\n";
}


Measured memory_bytes: 4976