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#pragma GCC optimize("Ofast")
#pragma GCC optimize("unroll-loops")
//#pragma GCC target("sse,sse2,sse3,ssse3,abm,mmx,tune=native")
//#pragma clang diagnostic ignored "-Wunused-const-variable"
#include<vector>
#include<iostream>
#include<stack>
#include<cmath>
#include<algorithm>
#include<set>
#include<map>
#include<string>
#include<tuple>
#include<bitset>
#include<queue>
#include<unordered_map>
#include<unordered_set>
#include<random>
#include<assert.h>
#include<ctime>
#include <cassert>
#include <thread>
#include <mutex>
#include <semaphore>
#include <chrono>
//#define int long long
#define all(a) a.begin(), a.end()
typedef long long ll;
typedef long double ld;
typedef unsigned long long ull;
using namespace std;
ll gcd(ll i, ll j) {
	if (i < j)swap(i, j);
	if (j == 0)return i;
	else return gcd(j, i % j);
}
#ifdef _MY
ll __builtin_popcount(ll x) { return x ? (__builtin_popcount(x >> 1) + (x & 1)) : 0; }
#endif
template<typename T> inline T getint() {
	T val = 0;
	char c;

	bool neg = false;
	while ((c = getchar()) && !(c >= '0' && c <= '9')) {
		neg |= c == '-';
	}

	do {
		val = (val * 10) + c - '0';
	} while ((c = getchar()) && (c >= '0' && c <= '9'));

	return val * (neg ? -1 : 1);
}
const ll INF = 1e7 + 100;
const int mod = 998244353, mod1 = 1000000007;
const ld eps = 1e-7, pi = acos(-1);
const int maxT = 503, maxN = 5'010, A = 311;
ll bp(ll et, ll b) {
	ll res = 1;
	for (int i = 30; i >= 0; --i) {
		res = (res * res) % mod;
		if ((b & (1 << i)) != 0)res = (res * et) % mod;
	}
	return res;
}
mt19937 mt_rand(time(0));
void panic() {
	cout << "No\n";
	exit(0);
}
ld get_time() {
	ll tmr = clock();
	return (ld)tmr / CLOCKS_PER_SEC;
}
int pl(int a, int b) {
	a += b;
	if (a >= mod)
		a -= mod;
	return a;
}
int mul(ll a, ll b) {
	return a * b % mod;
}
const ll sz = 1 << 19;
ll t[sz * 2];
ll mx_ans = 0;
ll get(int ps) {
	return t[ps];
}
void sett(int ps, ll val) {
	t[ps] = max(t[ps], val);
	mx_ans = max(mx_ans, val);
}
void solve() {
	ll n, c;
	cin >> n >> c;
	map<ll, vector<ll>>mp;
	for (int i = 0; i < n; ++i) {
		int a, b;
		cin >> a >> b;
		mp[a].push_back(b);
	}
	for (auto x : mp) {
		vector<pair<ll, ll>>vec_to_sett;
		for (auto col : x.second) {
			ll nans = 0;
			nans = max(nans, get(col) + x.first);
			nans = max(nans, x.first - c + mx_ans);
			vec_to_sett.emplace_back(col, nans);
		}

		for (auto ps : vec_to_sett) {
			sett(ps.first, ps.second);
		}
	}
	cout << mx_ans << "\n";
}
int32_t main()
{
	ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
	cout.precision(14);
#ifdef _MY
	freopen("input.txt", "r", stdin);
	freopen("output.txt", "w", stdout);


	/*time_t start, end;
	time(&start);*/
#else
	//freopen("epic.in", "r", stdin); freopen("epic.out", "w", stdout);
#endif
	//init();
	int t = 1;
	//cin >> t;
	while (t--) {
		solve();
	}
#ifdef _MY
	cout << "Time taken is : " << fixed << get_time() << " sec " << "\n";
#endif
	return 0;
}