#include<iostream>
#include<vector>
using namespace std;

int main(){
	int n;
	cin >> n;
	
	vector<int> input(n);
	for(int i = 0; i < n; i++) cin >> input[i];
	
	int ans = -1, sum_cnt = 1;
	for(int i = 0; i < n - 1; i++){
		if(input[i] + 1 == input[i + 1]) sum_cnt++;
		else ans = max(ans, sum_cnt), sum_cnt = 1;
	}
	
	cout << ans;
	return 0;
}