#include<bits/stdc++.h> using namespace std; int n,a,b,t=1,ans=0; int main(){ cin>>n>>a; for(int i=2;i<=n;i++){ cin>>b; if(b-a==1) t++; else t=1; if(t>ans) ans=t; a=b; } if(ans==1) ans=0; cout<<ans<<endl; return 0; }