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// Author: Olaf Surgut (surgutti)
// Created on 13-03-2025 18:16:45
#include "bits/stdc++.h"
using namespace std;

#define int long long
#define ll long long
#define ld long double

#define endl '\n'
#define st first
#define nd second
#define pb push_back
#define eb emplace_back
#define sz(x) (int)(x).size()
#define all(x) begin(x),end(x)
#define FOR(i,l,r) for(int i=(l);i<=(r);i++)
#define ROF(i,r,l) for(int i=(r);i>=(l);i--)

auto& operator<<(auto &o, pair<auto, auto> p) {
	return o << "(" << p.st << ", " << p.nd << ")";}
auto operator<<(auto &o, auto x)->decltype(end(x), o) {
	o << "{"; int i=0; for (auto e : x) o << ","+!i++ << e;
	return o << "}";}

#ifdef LOCAL
#define debug(x...) cerr << "[" #x "]: ", [](auto...$) { \
	((cerr << $ << "; "),...) << endl; }(x)
#else
#define debug(...)
#endif

#define rep(i,a,b) for(int i = a; i < (b); i++)
using pii = pair<int, int>;
using vi = vector<int>;

const int N = 500000 + 7;

int n, c, a[N], w[N];

int dp[N];

void solve() {
	cin >> n >> c;
	rep(i, 0, n)
		cin >> a[i] >> w[i];
	
	#define apply asfsdsdf

	vector<pii> apply;

	int last = a[0];
	rep(i, 0, n) {

		if (a[i] != last) {
			debug(apply);
			for (auto [k, j] : apply)
				dp[k] = max(dp[k], j);
			apply.clear();
		}

		int best = max(dp[w[i]] + a[i], dp[0] + a[i] - c);
		
		apply.eb(w[i], best);
		apply.eb(0, best);
	
		debug(i, best);

		last = a[i];
	}

	for (auto [i, j] : apply)
		dp[i] = max(dp[i], j);

	cout << *max_element(dp, dp + N) << '\n';
}

signed main() {
	cin.tie(0)->sync_with_stdio(0);

	int tt = 1;
	// cin >> tt;
	while (tt--)
		solve();

	return 0;
}