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/*
 * Opis: Główny nagłówek
 */
#include<bits/stdc++.h>
using namespace std;
using LL=long long;
#define FOR(i,l,r)for(int i=(l);i<=(r);++i)
#define REP(i,n)FOR(i,0,(n)-1)
#define ssize(x)int(x.size())
#ifdef DEBUG
auto&operator<<(auto&o,pair<auto,auto>p){return o<<"("<<p.first<<", "<<p.second<<")";}
auto operator<<(auto&o,auto x)->decltype(x.end(),o){o<<"{";int i=0;for(auto e:x)o<<","+!i++<<e;return o<<"}";}
#define debug(X...)cerr<<"["#X"]: ",[](auto...$){((cerr<<$<<"; "),...)<<endl;}(X)
#else
#define debug(...){}
#endif

struct Choice {
	int h;
	vector<int> wzo;
};

int main() {
	cin.tie(0)->sync_with_stdio(0);
	int n, c;
	cin >> n >> c;
	vector<pair<int, int>> arr(n);
	for (auto& [h, w] : arr) {
		cin >> h >> w;
	}
	vector<Choice> choices;
	int prv = -1;
	for (auto [h, w] : arr) {
		if (h != prv) {
			choices.emplace_back();
			choices.back().h = h;
			prv = h;
		}
		choices.back().wzo.push_back(w);
	}
	LL res = 0;
	map<int, LL> best_col;
	for (auto [h, w] : arr) {
		best_col[w] = 0;
	}
	for (const auto& choice : choices) {
		vector<pair<int,LL>> cand;
		for (int v : choice.wzo) {
			LL best = res - c + choice.h;
			best = max(best, best_col[v] + choice.h);
			cand.push_back({v, best});
		}
		for (auto [v, best] : cand) {
			res = max(res, best);
			best_col[v] = max(best_col[v], best);
		}
	}
	cout << res << "\n";



}