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#define _USE_MATH_DEFINES
#include <bits/stdc++.h>
#define deb(...)
#define DBP(...)
using namespace std;
using   ll         = long long;
using   vi         = vector<int>;
using   pii        = pair<int, int>;
#define pb           push_back
#define mp           make_pair
#define x            first
#define y            second
#define rep(i, b, e) for (int i = (b); i < (e); i++)
#define each(a, x)   for (auto& a : (x))
#define all(x)       (x).begin(), (x).end()
#define sz(x)        int((x).size())

void run();

int main() {
	cin.sync_with_stdio(0); cin.tie(0);
	cout << fixed << setprecision(12);
	run();
	cout << flush; _Exit(0);
}

constexpr int MX = 500'001;

void run() {
	int n, c;
	cin >> n >> c;

	vi a(n), w(n);
	rep(i, 0, n) cin >> a[i] >> w[i];

	vector<ll> dp(n), best(MX);
	ll ans = 0;

	for (int b = 0, e = 0; b < n; b = e) {
		for (; e < n && a[b] == a[e]; e++) {
			dp[e] = max(best[w[e]], ans-c) + a[e];
		}
		rep(i, b, e) {
			best[w[i]] = max(best[w[i]], dp[i]);
			ans = max(ans, dp[i]);
		}
	}

	cout << ans << '\n';
}