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#include <bits/stdc++.h>
using namespace std;

#define ll long long

int main() {
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    int n, c;
    cin >> n >> c;
    vector<pair<ll, ll>> p (n);
    for (int i = 0; i < n; i++) {
        cin >> p[i].first >> p[i].second;
    }
    sort(p.begin(), p.end());
    ll m = 0;
    unordered_map<ll, ll> mp;
    ll t = p[0].first;
    mp[p[0].second] = p[0].first;
    for (int i = 1; i < n; i++) {
        if (p[i].first > p[i-1].first) {
            m = t;
            ll x = max(mp[p[i].second]+p[i].first, m+p[i].first-c);
            mp[p[i].second] = x;
            t = max(t, x);
            continue;
        }
        if (p[i].second == p[i-1].second) {
            continue;
        }
        ll x = max(mp[p[i].second]+p[i].first, m+p[i].first-c);
        mp[p[i].second] = x;
        t = max(t, x);
    }
    cout << t << '\n';
}