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#include <bits/stdc++.h>
// #include <ext/pb_ds/assoc_container.hpp>
// #include <ext/pb_ds/tree_policy.hpp>
// #include <ext/rope>

using namespace std;
// using namespace __gnu_pbds;

typedef long long ll;
typedef unsigned long long ull;
typedef unsigned int uint;
typedef long double ld;

template <class T>

using VV = vector<vector<T>>;

using VI = vector<int>;
using VVI = vector<vector<int>>;

using VL = vector<long long>;
using VVL = vector<vector<long long>>;

using VC = vector<char>;
using VVC = vector<vector<char>>;

using VB = vector<bool>;
using VVB = vector<vector<bool>>;

using VD = vector<double>;
using VVD = vector<vector<double>>;

using PII = pair<int, int>;
using PLL = pair<long long, long long>;
using PIL = pair<int, long long>;
using PLI = pair<long long, int>;

using VPII = vector<pair<int, int>>;
using VPLL = vector<pair<long long, long long>>;

#define LINE '\n'
#define SPACE ' '
#define PB push_back
#define FOR(i, a, b) for (int i = (a); i < (int(b)); i++)
#define FORE(i, a, b) for (int i = (a); i <= (int)((b)); i++)
#define ALL(x) x.begin(), x.end()
#define RALL(x) x.rbegin(), x.rend()
#define sq(a) ((a) * (a))
#define sz(x) ((int)x.size())

#define fastio()                      \
    ios_base::sync_with_stdio(false); \
    cin.tie(nullptr);                 \
    cout.tie(nullptr)
#define debug(x) cerr << #x << " = " << x << endl;

const ll MOD = 1e9 + 7;

template <class T>
inline void maxi(T &a, T b)
{
    a = max(a, b);
}

template <class T>
inline void mini(T &a, T b)
{
    a = min(a, b);
}

template <class T>
inline void addi(T &a, T b)
{
    a = (a + b) % MOD;
}

template <class T>
inline void subi(T &a, T b)
{
    a = (a - b + MOD) % MOD;
}

template <class T>
inline T add(T a, T b)
{
    return (a + b) % MOD;
}

template <class T>
inline T sub(T a, T b)
{
    return (a - b + MOD) % MOD;
}

template <class T>
inline T mul(T a, T b)
{
    return ((a % MOD) * (b % MOD)) % MOD;
}

constexpr ll binpow(ll a, ll b, ll mod)
{
    ll res = 1;
    while (b > 0)
    {
        if (b & 1)
        {
            res = (res * a) % mod;
        }
        a = (a * a) % mod;
        b >>= 1;
    }

    return res;
}

const int INF = 1e9;

const int MAX_N = 5e5 + 3;

VI wzo[MAX_N];

ll dp = 0;
ll dp2[MAX_N];

void solve()
{
    int n, c;
    cin >> n >> c;

    VPII vec(n);
    FOR(i, 0, n)
    {
        int a, w;
        cin >> a >> w;

        vec[i] = {a, w};
        wzo[a].PB(w);
    }

    VI rozm;

    rozm.PB(vec[0].first);
    FOR(i, 1, n)
    {
        if (vec[i].first != vec[i - 1].first)
            rozm.PB(vec[i].first);
    }   
    reverse(ALL(rozm));

    ll ans = 0;

    FOR(i, 0, sz(rozm)) {
        ll a = rozm[i];

        
        ll fr = a + dp - c;
        maxi(ans, fr);

        VL vals;
        for(auto w : wzo[a]) {
            ll sc = a + dp2[w];
            maxi(ans, sc);
            vals.PB(sc);
        }

        int id = 0;
        for(auto w : wzo[a]) {
            maxi(dp2[w], max(fr, vals[id]));
            maxi(dp, vals[id]);
            id++;
        }

        
        maxi(dp, fr);
    }

    cout << ans << LINE;
}

int main()
{
    fastio();

    int t = 1;
    // cin >> t;

    while (t--)
        solve();
}