1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
#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

struct SegmentTree {
	vector<ll> _max;
	int base;
	SegmentTree(int n) : base(1) {
		while (base < n)
			base *= 2;
		_max = vector<ll>(2 * base, 0);
	}
	void put(int p, ll v) {
		p += base;
		_max[p] = max(_max[p], v);
		for (p /= 2; p > 0; p /= 2)
			_max[p] = max(_max[2 * p], _max[2 * p + 1]);
	}
	// [l, r]
	ll get(int l, int r) {
		if (l > r)
			return 0;
		l += base;
		r += base;
		ll res = max(_max[l], _max[r]);
		while (l / 2 != r / 2) {
			if (l % 2 == 0)
				res = max(res, _max[l + 1]);
			if (r % 2 == 1)
				res = max(res, _max[r - 1]);
			l /= 2;
			r /= 2;
		}
		return res;
	}
};

void sol() {
	int n;
	ll c;
	cin >> n >> c;
	vector<pair<int, int> > klocki(n);
	int max_st = 0, max_nd = 0;
	rep(i, n) {
		cin >> klocki[i].st >> klocki[i].nd;
		klocki[i].st -= 1;
		klocki[i].nd -= 1;
		max_st = max(max_st, klocki[i].st);
		max_nd = max(max_nd, klocki[i].nd);
	}
	vector<vector<pair<int, ll> > > typy(max_st + 1);
	rep(i, n) typy[klocki[i].st].pb({klocki[i].nd, 0});
	SegmentTree seg(max_nd + 1);
	rep(i, max_st + 1) {
		for (auto &[j, v] : typy[i]) {
			v = max(
				max(seg.get(0, j - 1), seg.get(j + 1, max_nd)) + (i + 1) - c,
				seg.get(j, j) + (i + 1));
		}
		for (auto [j, v] : typy[i]) {
			seg.put(j, v);
		}
	}
	cout << seg.get(0, max_nd) << '\n';
}

int32_t main() {
	cin.tie(0)->sync_with_stdio(0);
	cout << fixed << setprecision(10);
	sol();
#ifdef LOCF
	cout.flush();
	cerr << "- - - - - - - - -\n";
	(void)!system(
		"grep VmPeak /proc/$PPID/status | sed s/....kB/\' MB\'/1 >&2"); // 4x.kB
																		// ....kB
#endif
}