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#include <bits/stdc++.h>

using namespace std;

#define _DEBUG
#ifdef _DEBUG
template<typename T1, typename T2> auto& operator<<(ostream& o, pair<T1, T2> a) { return o << "(" << a.first << ", " << a.second << ")"; }
template<typename T, typename OS> auto& operator<<(OS& o, T a) { o << "{"; for (auto b : a) o << b << ", "; return o << "}"; }
#define dbg(x...) cerr << "[" #x "]: ", [](auto... args) { ((cerr << args << ", "),...) << "\n"; }(x)
#else
#define dbg(...)
#endif

#define sz(x) ((int)(x).size())
#define all(x) (x).begin(), (x).end()
#define F first
#define S second

using ll = long long;
using ld = long double;
using pll = pair<ll, ll>;
using vi = vector<int>;

int main() {
    ios::sync_with_stdio(false); cin.tie(nullptr);

    ll n, c;
    cin >> n >> c;
    vector<pll> v(n);
    for(ll i = 0 ; i < n ; ++i) cin >> v[i].F >> v[i].S;
    sort(v.begin(), v.end(), greater<>());
    map<ll,ll> mp;
    ll mx = 0;
    vector<pll> nxt;
    for(int i = 0 ; i < n ; ++i) {
        auto [p, q] = v[i];
        if(i>0 && p != v[i-1].F) {
            for(auto [x, y] : nxt) {
                mp[x] = max(mp[x], y);
                mx = max(mx, y);
            }
            nxt.clear();
        }

        ll cur = max(mp[q]+p, mx+p-c);
        nxt.emplace_back(q, cur);
    }
    for(auto [x, y] : nxt) mx = max(mx, y);
    for(auto [x, y] : mp) mx = max(mx, y);

    cout << mx << '\n';

    return 0;
}