#include<bits/stdc++.h> using namespace std; int n,a[10001],t=1,ans; //int li_si_xian_ri_shan_ben int main(){ cin>>n; for(int i=1;i<=n;i++){ cin>>a[i]; if(i==1) continue; if(a[i]-a[i-1]==1) t++; else t=1; ans=max(ans,t); } if(ans==1) ans=0; cout<<ans; return 0; }