const int LG = 21;
const int FN = 400005;
const long long MOD = 1e9 + 7;
const long long INF = 1e9;
const long long INFLL = 1e18;

#include <iostream>
#include <vector>
#include <string>
#include <map>
#include <set>
#include <algorithm>
#include <cmath>
#include <time.h>
#include <unordered_set>
#include <bitset>
#include <deque>

using namespace std;

typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef vector<ll> vi;

#define forn(i, n) for (int (i) = 0; (i) != (n); (i)++)
#define all(v) (v).begin(), (v).end()
#define rall(v) (v).rbegin(), (v).rend()
#define popcount(x) __builtin_popcount(x)
#define popcountll(x) __builtin_popcountll(x)
#define fi first
#define se second
#define re return
#define uniq(x) sort(all(x)); (x).resize(unique(all(x)) - (x).begin())

#ifdef LOCAL
#define dbg(x) cerr << __LINE__ << " " << #x << " " << x << endl
#define ln cerr << __LINE__ << endl
#else
#define dbg(x) void(0)
#define ln void(0)
#endif // LOCAL

int cx[4] = {-1, 0, 1, 0};
int cy[4] = {0, -1, 0, 1};

ll inq(ll x, ll y)
{
    if (!y) re 1 % MOD;
    ll l = inq(x, y / 2);
    if (y % 2) re l * l % MOD * x % MOD;
    re l * l % MOD;
}

ll rev(ll x)
{
    return inq(x, MOD - 2);
}

bool __precomputed_combinatorics = 0;
vector<ll> __fact, __ufact, __rev;

void __precompute_combinatorics()
{
    __precomputed_combinatorics = 1;
    __fact.resize(FN);
    __ufact.resize(FN);
    __rev.resize(FN);
    __rev[1] = 1;
    for (int i = 2; i < FN; i++) __rev[i] = MOD - __rev[MOD % i] * (MOD / i) % MOD;
    __fact[0] = 1, __ufact[0] = 1;
    for (int i = 1; i < FN; i++) __fact[i] = __fact[i - 1] * i % MOD, __ufact[i] = __ufact[i - 1] * __rev[i] % MOD;
}

ll fact(int x)
{
    if (!__precomputed_combinatorics) __precompute_combinatorics();
    return __fact[x];
}

ll cnk(int n, int k)
{
    if (!__precomputed_combinatorics) __precompute_combinatorics();
    return __fact[n] * __ufact[n - k] % MOD * __ufact[k] % MOD;
}

const int N = 100500;

int n;
ll L;
ll x[N], v[N];
ll pv[N], sv[N];
ll prefbest1[N];
ll prefbest2[N];
ll suffbest1[N];
ll suffbest2[N];

signed main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);
    cin >> n >> L;
    forn(i, n) cin >> x[i] >> v[i];
    ll ans = 0;
    pv[0] = v[0];
    for (int i = 1; i < n; i++) pv[i] = pv[i - 1] + v[i];
    sv[n - 1] = v[n - 1];
    for (int i = n - 2; i >= 0; i--) sv[i] = sv[i + 1] + v[i];

    for (int i = 0; i < n; i++)
    {
        prefbest1[i] = max(0LL, pv[i] - x[i]);
        prefbest2[i] = max(0LL, pv[i] - 2 * x[i]);
        if (i) prefbest1[i] = max(prefbest1[i], prefbest1[i - 1]);
        if (i) prefbest2[i] = max(prefbest2[i], prefbest2[i - 1]);
    }
    for (int i = 0; i < n; i++)
    {
        suffbest1[i] = sv[i] - (L - x[i]);
        suffbest2[i] = sv[i] - 2 * (L - x[i]);
        suffbest1[i] = max(suffbest1[i], suffbest1[i + 1]);
        suffbest2[i] = max(suffbest2[i], suffbest2[i + 1]);
    }
    ans = max(ans, suffbest1[0]);
    ans = max(ans, prefbest1[n - 1]);
    for (int i = 0; i + 1 < n; i++)
    {
        ans = max(ans, prefbest1[i] + suffbest2[i + 1]);
        ans = max(ans, prefbest2[i] + suffbest1[i + 1]);
    }
    cout << ans;
}

/* Note:
Check constants at the beginning of the code.
    N is set to 4e5 but be careful in problems with large constant factor.
    Setting N in every problem is more effective.
Check corner cases.
    N = 1
No def int long long for now.
Add something here.
*/


Measured memory_bytes: 9940