#include<bits/stdc++.h>
using namespace std;
int n,ans = 0,sum = 1,m,temp; 
int main(){
	cin >> n >> m;
	temp = m;
	for(int i = 1;i < n;i ++){
		cin >> m;
		if(temp + 1 == m){
			sum ++;
		}else{
			ans = max(ans,sum);
			sum = 1;
		}
		temp = m;
	}
	cout << ans << endl;
	return 0;
}