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#include <iostream>
#include <vector>
#include <set>
#include <map>
#include <algorithm>
#include <unordered_map>
#include <unordered_set>
#include <queue>
#include <deque>
#include <tuple>
#include <bitset>

using namespace std;

#define pb push_back
#define mp make_pair
#define st first
#define nd second
#define x first
#define y second

typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef vector<int> vi;
typedef vector<ll> vll;

ll best_color[500001];
ll best_all=0;

int main(){
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    
    int n, c;
    
    cin >> n >> c;

    map<ll, set<ll>> boxes;

    for(int i=0; i<n; i++){
        ll a, w;
        cin >> a >> w;
        boxes[a].insert(w);
    }

    ll ans = 0;

    for (auto& a: boxes) {
        vector<pll> size_dp;

        for(auto w: a.nd){
            ll dp = a.st + max(0ll, max(best_all - c, best_color[w]));
            ans = max(ans, dp);
            size_dp.pb(mp(dp, w));
        }
    
        for(auto dp: size_dp){
            best_all = max(best_all, dp.st);
            best_color[dp.nd] = max(best_color[dp.nd], dp.st);
        }
    }

    cout << ans;
    return 0;
}