#include<bits/stdc++.h>
using namespace std;
#define rep(i,n) for(ll i=0;i<n;i++)
#define repl(i,l,r) for(ll i=(l);i<(r);i++)
#define per(i,n) for(ll i=(n)-1;i>=0;i--)
#define perl(i,r,l) for(ll i=r-1;i>=l;i--)
#define fi first
#define se second
#define pb push_back
#define ins insert
#define pqueue(x) priority_queue<x,vector<x>,greater<x>>
#define all(x) (x).begin(),(x).end()
#define CST(x) cout<<fixed<<setprecision(x)
#define vtpl(x,y,z) vector<tuple<x,y,z>>
#define rev(x) reverse(x);
using ll=long long;
using vl=vector<ll>;
using vvl=vector<vector<ll>>;
using pl=pair<ll,ll>;
using vpl=vector<pl>;
using vvpl=vector<vpl>;
const ll MOD=1000000007;
const ll MOD9=998244353;
const int inf=1e9+10;
const ll INF=4e18;
const ll dy[9]={0,1,-1,0,1,1,-1,-1,0};
const ll dx[9]={1,0,0,-1,1,-1,1,-1,0};
template<class T> inline bool chmin(T& a, T b) {
    if (a > b) {
        a = b;
        return true;
    }
    return false;
}
template<class T> inline bool chmax(T& a, T b) {
    if (a < b) {
        a = b;
        return true;
    }
    return false;
}
int main(){
    ll n;cin >> n;
    vl a(n);rep(i,n)cin >> a[i];
    ll ng=-1,ok=INF;
    rep(i,n)chmax(ng,a[i]);ng--;
    while(ok-ng>1){
        ll mid=(ok+ng)/2;
        vl v(n);
        rep(i,n)v[i]=mid-a[i];
        ll dp[n][n][2];//区間[j,j+i]を埋めていて、0なら左端、1なら右端にある
        rep(i,n)rep(j,n)rep(k,2)dp[i][j][k]=INF;
        rep(i,n)dp[0][i][0]=0;
        rep(i,n-1){
            rep(j,n){
                ll right=i+j;
                if(j&&v[j-1]>=dp[i][j][0]+1)chmin(dp[i+1][j-1][0],dp[i][j][0]+1);
                if(right+1<n&&v[right+1]>=dp[i][j][0]+i+1)chmin(dp[i+1][j][1],dp[i][j][0]+i+1);
                if(j&&v[j-1]>=dp[i][j][1]+i+1)chmin(dp[i+1][j-1][0],dp[i][j][1]+i+1);
                if(right+1<n&&v[right+1]>=dp[i][j][1]+1)chmin(dp[i+1][j][1],dp[i][j][1]+1);
            }
        }
        if(dp[n-1][0][0]!=INF||dp[n-1][0][1]!=INF)ok=mid;
        else ng=mid;
    }
    cout << ok << endl;
}