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#include <bits/stdc++.h>
using namespace std;

#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

#ifdef DEBUG
constexpr int N = 10;
#else
constexpr int N = int(5e5 + 10);
#endif

int main() {
	ios_base::sync_with_stdio(0);
	cin.tie(0);

	int n, c;
	cin >> n >> c;

	vector<long long> max_bucket(N, 0);

	long long answer = 0;
	long long best = 0;
	int qa = -1;
	vector<pair<long long, int>> que;

	REP (i, n) {
		int a, w;
		cin >> a >> w;

		if (!que.empty() and a > qa) {
			for (auto [a1, w1] : que) {
				best = max(best, a1);
				max_bucket[w1] = max(max_bucket[w1], a1);
			}

			que.clear();
		}

		qa = a;

		long long before = 0;
		before = max(max_bucket[w], best - c);

		long long dp = a + before;
		answer = max(answer, dp);
		que.emplace_back(dp, w);
	}

	cout << answer << '\n';
}