#include <bits/stdc++.h>   using namespace std;   int main() {     {       int n, m;       cin >> n >> m;       vector<int> a(n + 2);       for (int i = 1; i <= n; i++) cin >> a[i];       a[0] = 0, a[n + 1] = m;       vector<int> light(n + 2, 0), dark(n + 2, 0);       for (int i = 1; i <= n + 1; i++) {         light[i] = light[i - 1], dark[i] = dark[i - 1];         if (i % 2)           light[i] += a[i] - a[i - 1];         else           dark[i] += a[i] - a[i - 1];       }       int ans = light[n + 1];       for (int i = 1; i <= n + 1; i++) {         ans =             max(ans, light[i - 1] + dark[n + 1] - dark[i] + a[i] - a[i - 1] - 1);       }       cout << ans << endl;     }   }   