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#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define vc vector
#define st first
#define nd second
#define pll pair<ll, ll>
#define sz(a) (ll)a.size()
#define all(a) a.begin(), a.end()
#define pu push
#define pub push_back
#define pob pop_back
#define em emplace
#define emb emplace_back
ll MAX = 500'005;

void program() {
	ll n, c;
	cin >> n >> c;
	vc<ll> a(n), w(n);
	for (ll i = 0; i < n; i++)
		cin >> a[i] >> w[i];
	
	vc<vc<ll>> b(MAX);
	for (ll i = 0; i < n; i++)
		b[a[i]].pub(w[i]);
	
	ll dpg = 0;
	vc<ll> dpw(MAX, 0);
	for (ll j = 0; j < MAX; j++) {
		auto &l = b[j];
		ll m = sz(l);
		vc<ll> dp(m, 0);
		for (ll i = 0; i < m; i++)
			dp[i] = max(dpg - c, dpw[l[i]]);
		for (ll i = 0; i < m; i++) {
			dpg = max(dpg, dp[i] + j);
			dpw[l[i]] = max(dpw[l[i]], dp[i] + j);
		}
	}
	cout << dpg << "\n";
}	

int main() {
	ios_base::sync_with_stdio(false);
	cin.tie(nullptr);
	cout.tie(nullptr);
	program();
	return 0;
}