func_code_string: #include <bits/stdc++.h>   using namespace std;   long long a[100010], b[100010], x[100010], y[100010];   int main() {     ios::sync_with_stdio(false);     cin.tie(0);     long long n, m, ans = 0;     cin >> n >> m;     for (int i = 1; i <= n; i++) cin >> a[i];     a[0] = 0;     a[n + 1] = m;     for (int i = 1; i <= n + 1; i++) {       b[i] = a[i] - a[i - 1];       if (i & 1) ans += b[i];       x[i] = x[i - 1];       y[i] = y[i - 1];       if (i & 1)         x[i] += b[i];       else         y[i] += b[i];     }     for (int i = 1; i <= n + 1; i++) {       if (b[i] > 1) {         ans = max(ans, x[i - 1] + y[n + 1] - y[i] + b[i] - 1);       }     }     cout << ans;     return 0;   }   