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

#define ll long long

const int MAXN = 5e5 + 7;
int kol[MAXN];
ll dp[MAXN];

int main(){
    int n, c;
    cin >> n >> c;

    vector<pair<ll, int>> klocki(n);

    for(int i = 0; i < n; i++){
        cin >> klocki[i].first >> klocki[i].second;
    }
    sort(klocki.begin(), klocki.end());
    fill(kol, kol + MAXN, MAXN);

    int idx_best = MAXN - 1, idx_act = MAXN - 1;
    ll wyn = 0;
    for(int i = n - 1; i >= 0; i--){
        if(i + 1 != n && klocki[i] == klocki[i + 1]){
            dp[i] = dp[i + 1];
            continue;
        }

        if(klocki[i + 1].first != klocki[i].first) if(dp[idx_best] < dp[idx_act]) idx_best = idx_act;
        // cerr << "i: " << i << '\n';
        dp[i] = klocki[i].first;
        // cerr << "klocki[i]: " << klocki[i].first << ' ' << klocki[i].second << '\n';
        // cerr << "ok\n";
        // cerr << "kol[klocki[i].second]: " << kol[klocki[i].second] << '\n';
        // if(klocki[kol[klocki[i].second]] == klocki[i]) dp[i] = max(dp[i], dp[i + 1]);
        if (kol[klocki[i].second] != MAXN) dp[i] = max(dp[i], dp[kol[klocki[i].second]] + klocki[i].first);
        // cerr << "ok\n";

        dp[i] = max(dp[i], dp[idx_best] - c + klocki[i].first);
        if(dp[idx_act] < dp[i]) idx_act = i;
        kol[klocki[i].second] = i;
        wyn = max(dp[i], wyn);
    }

    // for(int i = 0; i < n; i++) cerr << dp[i] << ' ';
    // cerr << '\n';

    cout << wyn << '\n';
}