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#include <bits/stdc++.h>
using namespace std;
#define fwd(i, a, n) for (int i = (a); i < (n); i++)
#define rep(i, n) fwd(i, 0, n)
#define all(X) X.begin(), X.end()
#define sz(X) int(size(X))
#define pb push_back
#define eb emplace_back
#define st first
#define nd second
using pii = pair<int, int>; using vi = vector<int>;
using ll = long long; using ld = long double;
#ifdef LOC
auto SS = signal(6, [](int) { *(int *)0 = 0; });
#define DTP(x, y) auto operator << (auto &o, auto a) -> decltype(y, o) { o << "("; x; return o << ")"; }
DTP(o << a.st << ", " << a.nd, a.nd);
DTP(for (auto i : a) o << i << ", ", all(a));
void dump(auto... x) { (( cerr << x << ", " ), ...) << '\n'; }
#define deb(x...) cerr << setw(4) << __LINE__ << ":[" #x "]: ", dump(x)
#else
#define deb(...) 0
#endif

#define S 500007

ll dp[S];
ll pref[S];

vector<int> V[S];
int H[S];

int32_t main() {
	cin.tie(0)->sync_with_stdio(0);
	cout << fixed << setprecision(10);
	int n,c;
	cin >> n >> c;

	int l = 0;

	for(int i = 1; i <= n; i++) {
		ll a,w;
		cin >> a >> w;
		H[i] = a;
		while(l + 1 < i && H[l+1] < H[i]) {
			l++;
		}

		dp[i] = a;
		dp[i] = max(dp[i], pref[l] + a - c);

		if(!V[w].empty()) {
			int v = V[w].back();
			if(H[v] == H[i]) {
				dp[i] = max(dp[i], dp[v]);
			} else {
				dp[i] = max(dp[i], dp[v] + a);
			}
		}
		V[w].pb(i);

		pref[i] = max(pref[i-1], dp[i]);
	}	
	cout << pref[n] << "\n"; 
}