// C++ code to find count of largest consectutive subset
#include <iostream>
#include <stdlib.h>
#include <unordered_set>
using namespace std;

int main() {
    int sz, num;
    unordered_set<int> numbers;
    int maxLen = 0;
    cin>>sz;
    for (int i=0;i<sz;i++) {
        cin>>num;
        numbers.insert(num);
    }
    
    for (int i : numbers) {
        if (numbers.find(i-1) == numbers.end()) {
            int temp = i;
            while (numbers.find(temp) != numbers.end()) temp++;
            
            if (maxLen < (temp-i)) maxLen = temp-i;
        }
    }

    cout<< maxLen;
    return 0;
}