#include <bits/stdc++.h>   using namespace std;   int main() {     int n;     cin >> n;     int arr[201];     for (int i = 1; i <= n; ++i) cin >> arr[i];     int brr[201] = {0}, count = 0;     for (int i = 1; i <= n; ++i) {       if (i == 1 && n >= 2) {         if (arr[1] == 1 && arr[2] == 2) brr[i] = 1;       } else {         if (i == n && n >= 2) {           if (arr[n] == 1000 && arr[n - 1] == 999) brr[i] = 1;         } else {           if (arr[i - 1] == arr[i] - 1 && arr[i + 1] == arr[i] + 1) brr[i] = 1;         }       }     }     int t = 0;     for (int i = 1; i <= n;) {       while (i <= n && brr[i] == 1) {         t++;         i++;       }       while (i <= n && brr[i] == 0) {         i++;       }       count = max(count, t);       t = 0;     }     cout << count << endl;   }   