#include <bits/stdc++.h>

using namespace std;
using ll = long long;

struct event {
    int x;
    int y1, y2;
    bool open;
};

struct point {
    int x, y;
};

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

    int N, M;
    cin >> N >> M;
    point A[N];
    for (int i = 0; i < N; i++) {
        cin >> A[i].x >> A[i].y;
    }
    vector<event> E;
    for (int i = 0; i < N; i++) {
        int j = (i + 1) % N;
        if (A[i].x != A[j].x) {
            continue;
        }
        if (A[i].y > A[j].y) {
            E.push_back(event{
                    A[i].x, A[j].y, A[i].y, false
            });
        } else {
            E.push_back(event{
                    A[i].x, A[i].y, A[j].y, true
            });
        }
    }
    sort(E.begin(), E.end(), [&](event a, event b) {
        return a.x > b.x;
    });
    if (!E.back().open) {
        for (int i = 0; i < E.size(); i++) {
            E[i].open ^= true;
        }
    }
    int ans = 0;
    int x = 0;
    set<pair<int, int>> X;
    set<pair<int, int>> O;
    vector<pair<int, int>> C;
    while (x <= M) {
        if (!E.empty()) {
            int x1 = E.back().x;
            if ((x1 % 2) != (x % 2)) {
                x1--;
            }
            x1 -= 2;
            if (x1 >= x) {
                x = x1;
            }
        }
        if (X.empty()) {
            ans = x;
        }
        auto bad = [&](pair<int, int> a) -> bool {
            return (a.first % 2) != (a.second % 2);
        };
        int cnt = 0;
        while (!E.empty()) {
            if (E.back().x != x) {
                break;
            }
            event e = E.back();
            E.pop_back();
            if (e.open) {
                int y1 = e.y1;
                int y2 = e.y2;
                while (true) {
                    auto it = O.lower_bound({y1, y2});
                    if (it != O.end()) {
                        if (it->first == y2) {
                            y2 = it->second;
                            if (bad(*it)) {
                                cnt--;
                            }
                            O.erase(it);
                            continue;
                        }
                    }
                    if (it != O.begin()) {
                        it--;
                        if (it->second == y1) {
                            y1 = it->first;
                            if (bad(*it)) {
                                cnt--;
                            }
                            O.erase(it);
                            continue;
                        }
                    }
                    break;
                }
                if (bad({y1, y2})) {
                    cnt++;
                }
                O.insert({y1, y2});
            } else {
                C.push_back({e.y1, e.y2});
            }
        }
        bool ok = true;
        for (auto c: C) {
            auto it = O.lower_bound({c.first + 1, -1});
            if (it != O.begin()) {
                it--;
                auto o = *it;
                if (o.first <= c.first && c.second <= o.second) {
                    pair<int, int> a = {o.first, c.first};
                    pair<int, int> b = {c.second, o.second};
                    if (bad(o)) {
                        cnt--;
                    }
                    O.erase(it);
                    if (a.first < a.second) {
                        if (bad(a)) {
                            cnt++;
                        }
                        O.insert(a);
                    }
                    if (b.first < b.second) {
                        if (bad(b)) {
                            cnt++;
                        }
                        O.insert(b);
                    }
                } else {
                    ok = false;
                    break;
                }
            } else {
                ok = false;
                break;
            }
        }
        if (!ok) {
            break;
        }
        if (cnt != 0) {
            break;
        }

        swap(O, X);
        C.clear();
        x++;
    }
    cout << ans << "\n";
    return 0;
}