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#include<iostream>
#include<vector>
#include<map>
#include<queue>
#include<algorithm>
using namespace std;

struct tower {
    long long cost;
    long long color;
};

bool operator<(const tower &a, const tower &b) {
    if (b.cost > a.cost) return true;
    if (b.cost < a.cost) return false;
    if (a.color > b.color) return true;
    return false;
}

int main() {
    ios_base::sync_with_stdio(0);
    cin.tie(0);

    int n, c;
    cin >> n >> c;

    map<long long, vector<long long>, greater<long long>> m;
    for (int i = 0; i < n; i++) {
        long long a, w;
        cin >> a >> w;
        
        m[a].push_back(w);
    }
    
    long long mx = 0;
    priority_queue<tower> pq;
    vector<long long> W(500'001, -1);
    for (const pair<long long, vector<long long>> &e : m) {
        const vector<long long> &v = e.second;
        vector<long long> z;
        for (long long color : v) {
            long long best = 0;
            if (W[color] != -1) {
                best = max(best, W[color]);
            }
            if (! pq.empty()) {
                tower t = pq.top();
                if (t.color == color) {
                    best = max(best, t.cost);
                }
                else {
                    best = max(best, t.cost - c);
                }
            }
            best += e.first;
            z.push_back(best);

            mx = max(mx, best);
        }
        for (int i = 0; i < v.size(); i++) {
            long long cost = z[i];
            long long color = v[i];
            W[color] = cost;
            pq.push({cost, color});
        }
    }
    cout << mx << endl;

    return 0;
}