#include <bits/stdc++.h>
using namespace std;
const int N = 1009;
int a[N][N];
int main() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
int n, m;
cin >> n >> m;
vector<int> X(n), Y(n);
for (int i = 0; i < n; i++) {
cin >> X[i] >> Y[i];
}
for (int i = 0; i < n; i++) {
int j = (i - 1 + n) % n;
if (X[i] == X[j]) {
int l = min(Y[i], Y[j]);
int r = max(Y[i], Y[j]);
for (int y = l; y < r; y++) {
a[X[i]][y] ^= 1;
}
}
}
for (int i = 1; i < N; i++) {
for (int j = 0; j < N; j++) {
a[i][j] ^= a[i - 1][j];
}
}
int ans = 0;
for (int i = 0; i < m; i++) {
bool ok = true;
bool one = false;
for (int j = 0; j < N - 1; j++) {
if (a[i][j]) {
one = true;
if (a[i][j + 1] && a[i + 1][j] && a[i + 1][j + 1]) {
a[i][j] = a[i + 1][j + 1] = a[i + 1][j] = a[i][j + 1] = 0;
} else {
ok = false;
break;
}
}
}
if (!ok) {
break;
}
if (!one) {
ans = i + 1;
}
}
cout << ans << '\n';
return 0;
}
