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#include <iostream>
#include <vector>
#include <map>

typedef long long ll;

using namespace std;

ll vmax(const vector<ll> &nums) {
    ll m = 0;
    for (ll num: nums) {
        m = max(num, m);
    }
    return m;
}

int main() {
    ios_base::sync_with_stdio(false);

    ll n, c;
    cin >> n >> c;

    vector<ll> a(n);
    vector<ll> w(n);
    vector<map<ll, ll> > groups;
    map<ll, ll> best;
    ll m = -1;

    ll prev = -1;
    for (int i = 0; i < n; i++) {
        cin >> a[i] >> w[i];
        if (a[i] != prev) {
            groups.emplace_back();
            prev = a[i];
        }
        groups[groups.size() - 1][w[i]] = i;
        best[w[i]] = 0;
    }

    for (const auto &group: groups) {
        vector<ll> candidates;
        candidates.push_back(m);

        for (auto [w,i]: group) {
            best[w] = max(best[w] + a[i], m + a[i] - c);
            candidates.push_back(best[w]);
        }
        m = vmax(candidates);
    }
    cout << m << endl;
    return 0;
}