#include<bits/stdc++.h>
using namespace std;
int a[121314];
int ans,n;
int main(){
	cin >> n;
	a[0] = -99;
	for(int i = 1,t = 0;i <= n;i ++){
		cin >> a[i];
		if(a[i] == a[i-1]+1) t++,ans = max(ans,t+1);
		else t = 0;
	}
	cout << ans;
}