#include<bits/stdc++.h>
using namespace std;
#ifndef BADGNU
#pragma GCC target("sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,avx2,tune=native")
#endif
#pragma GCC optimize("Ofast,unroll-loops,fast-math,O3")
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
#define ll long long
#define int ll
#define ld long double
#define y1 cheza
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
template<class T> using ordered_set = tree<T,null_type,less<T>,rb_tree_tag,tree_order_statistics_node_update>;
template<class T> using ordered_multiset = tree<T,null_type,less_equal<T>,rb_tree_tag,tree_order_statistics_node_update>;
const int N=305;
const int M=5001;
const int B=447;
const int mod=998244353;
const ll INF=2e9;
const int dx[]={1,-1,0,0};
const int dy[]={0,0,1,-1};
const double eps=1e-6;
int n;
int p[N];
int a[N];
int dp[4][N];
void test(){
cin>>n;
for(int i=0;i<n;i++){
cin>>p[i];
p[i]--;
}
int ans=n-1;
while(1){
vector<tuple<int,int,int>>v;
for(int mask=0;mask<4;mask++){
for(int i=0;i<n;i++){
if(dp[mask][i]!=INF){
int x=i-(mask&2?dp[mask][i]:0);
int y=p[i]-(mask&1?dp[mask][i]:0);
v.push_back({x,y,dp[mask][i]});
}
}
}
bool ok=1;
for(int mask=0;mask<4;mask++){
for(int i=0;i<n;i++){
dp[mask][i]=INF;
int cur=INF;
for(auto [x,y,w]:v){
x=(mask&2?i-(x+w):x-i);
y=(mask&1?p[i]-(y+w):y-p[i]);
if(x>0&&y>0){
cur=min(cur,max(x,y)+w);
}
}
int lx=(mask&2?i:(n-1)-i);
int rx=(mask&1?p[i]:(n-1)-p[i]);
if(cur<=min(lx,rx)){
dp[mask][i]=cur;
}
if(dp[mask][i]!=INF){
ok=0;
}
}
}
if(ok)break;
ans--;
}
cout<<ans<<'\n';
}
/*
*/
signed main(){
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
// cout.tie(nullptr);
int t2=1;
// cin>>t2;
for(int i=1;i<=t2;i++){
test();
}
}
