#include <iostream>
#include <cstdio>
#include <cstring>
#include <cmath>
#include <iomanip>
using namespace std;
int main(void){
	int n;
	int i;
	long long a[10000];
	cin>>n;
	for(i=0;i<n;i++){
		cin>>a[i];
	}
	int count=0;
	int max=0;
	for(i=0;i<n;i++){
		if(a[i]=a[i+1]-1){
			count++;
		}else{
			count=0;	
		}
		if(count>max){
			max=count;
		}
	}
	if(count!=0){
		cout<<max+1<<endl;
	}else{
		cout<<0<<endl;
	}
    return 0;
}