#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
int main(){
  ll n;
  cin>>n;
  ll a[n];
  for(ll i=0;i<n;i++) cin>>a[i];
  ll ans=1e18;
  for(ll x=0;x<2;x++){
    for(ll i=0;i<n;i++){
      ll t=0,u=0,v=ans;
      for(ll j=0;j<i;j++) t=max(t,a[j]-i-j);
      ll b[n+1];
      memset(b,0,sizeof(b));
      b[n]=t;
      //cout<<i<<":"<<t<<endl;
      for(ll j=n-1;j>=i;j--)
	b[j]=max(b[j+1],a[j]-((n-1)*2+i-j));
      //v=(n-1)*2+b[i];
      //cout<<v<<endl;
      u=t;
      for(ll j=i;j<n;j++){
	v=min(v,max(a[j],max(u,b[j+1])+(n-1)*2+i-j));
	//cout<<i<<" "<<j<<":"<<max(u,b[j+1])<<" "<<v<<" "<<+(n-1)*2+i-j<<endl;
	u=max(u,a[j]-i-j);
      }
      ans=min(ans,v);
    }
    reverse(a,a+n);
  }
  cout<<ans<<endl;
  return 0;
}