#include <bits/stdc++.h>
using namespace std;

int main() {    
    int k;
    while (cin >> k) {
        vector<int> a(k);
        for (int i = 0; i < k; i++)
            cin >> a[i];            
        long long curLen = 1;   // 当前连续子序列长度    
        long long maxLen = 1;   // 全局最大长度    
        for (int i = 1; i < k; i++) {
            if (a[i] - 1 == a[i-1])
                curLen += 1;
   			else
                curLen = 1;           		
            if (curLen > maxLen) 
                maxLen = curLen;
        }        
        cout << maxLen << endl;
    }    
    return 0;
}