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

int main(){
	long long tmp,a[11000],j = 0,n,len = 0,maxn = 0;
	cin >> n >> tmp;
	j = tmp;
	len = 1;
	for(int i = 2;i <= n;i++){
		cin >> a[i];
	}
	for(int i = 2;i <= n;i++){
		tmp = a[i];
		if(tmp == j+1){
			len++;
		}else{
			if (len > maxn)    maxn = len;
			len = 1;
		}
		j = tmp;
	}
	cout << maxn;
	return 0;
}