#include<bits/stdc++.h>
#define L(i, j, k) for(int i = (j); i <= (k); ++i)
#define R(i, j, k) for(int i = (j); i >= (k); --i)
#define ll long long
#define sz(a) ((int) (a).size())
#define vi vector < int >
#define me(a, x) memset(a, x, sizeof(a))
#define ull unsigned long long
#define ld __float128
#define pb emplace_back
using namespace std;
const int N = 1e6 + 7;
map<int,vector<pair<int,int>>>mp;
int n, m;
int x[N], y[N];
int arr[N], tot;
vector<pair<int,vector<pair<int,int>>>>vc;
bool hos[N], occ[N];
int main() {
	ios :: sync_with_stdio(false);
	cin.tie(0); cout.tie(0);
	cin >> n >> m;
	L(i, 1, n) {
		cin >> x[i] >> y[i];
		++y[i];
		arr[++tot] = y[i];
		arr[++tot] = y[i] ^ 1;
		arr[++tot] = y[i] + 2;
		arr[++tot] = (y[i] ^ 1) + 2;
	}
	sort(arr + 1, arr + tot + 1);
	tot = unique(arr + 1, arr + tot + 1) - arr - 1;
	L(i, 1, n) {
		y[i] = lower_bound(arr + 1, arr + tot + 1, y[i]) - arr;
	}
	L(i, 1, n) {
		int nxt = i % n + 1;
		if(x[nxt] == x[i]) {
			mp[x[i]].pb(min(y[i], y[nxt]), max(y[i], y[nxt]));
		}
	}
	for(auto&u : mp) {
		vc.pb(u);
	}
	int ns = 0;
	L(i, 0, sz(vc) - 2) {
		int len = vc[i + 1].first - vc[i].first;
		if(len >= 3) len = len % 2 + 2;
		for(auto&it : vc[i].second) {
			L(j, it.first + 1, it.second) {
				occ[j] ^= 1;
			}
		}
		L(t, 1, len) {
			int ban = 0;
			int all_zero = 1;
			vi diff;
			int lst = 0, var = 0;
			L(j, 1, tot) {
				if(occ[j]) {
					hos[j] ^= 1;
					all_zero &= !hos[j];
					if(var == 0) lst = hos[j];
					else if(lst != hos[j]) ban = 1;
					var ^= 1;
				}
				if(!occ[j] && var) {
					ban = 1;
				}
			}
			if(var)ban = 1;
			if(ban) {
				cout << ns << '\n';
				return 0;
			}
			if(all_zero) {
				ns = max(ns, vc[i + 1].first - len + t);
			}
		}
	}
	cout << ns << '\n';
	return 0;
}