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#include <bits/stdc++.h>

using namespace std;

#define rep(i, a, b) for (int i = a; i < (b); ++i)
#define ALL(x) begin(x), end(x)
#define SZ(x) (int)(x).size()
#define st first
#define nd second
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef vector<int> vi;
typedef vector<ll> vl;

#ifdef LOCAL
#define DTP(x, y)                                    \
    auto operator<<(auto &o, auto a)->decltype(y, o) \
    {                                                \
        o << "(";                                    \
        x;                                           \
        return o << ")";                             \
    }
DTP(o << a.first << ", " << a.second, a.second);
DTP(for (auto i : a) o << i << ", ", ALL(a));
void dump(auto... x) { ((cerr << x << ", "), ...) << '\n'; }
#define debug(x...) cerr << setw(4) << __LINE__ << ":[" #x "]: ", dump(x)
#else
#define debug(...) 42
#endif

void solve()
{
    int n;
    ll c;
    cin >> n >> c;
    vl D(500000, 0);
    ll last = -1, best = 0;
    queue<pll> Q;
    for (int i = 0; i < n; i++)
    {
        ll a, b;
        cin >> a >> b;
        --b;
        if (a != last)
        {
            while (Q.size())
            {
                D[Q.front().first] = max(D[Q.front().first], Q.front().second);
                best = max(best, Q.front().second);
                Q.pop();
            }
        }
        last = a;
        ll r = max(D[b] + a, best + a - c);
        // debug(r);
        Q.push({b, r});
    }
    while (Q.size())
    {
        best = max(best, Q.front().second);
        Q.pop();
    }
    cout << best << endl;
}

main()
{
    cin.tie(0)->sync_with_stdio(0);
    cin.exceptions(cin.failbit);

    int Z = 1;
    // cin >> Z;
    while (Z--)
        solve();
}