#include <bits/stdc++.h>
#define ll long long
#define INF 1000000005
#define MOD 1000000007
#define EPS 1e-10
#define rep(i,n) for(int i=0;i<(int)n;++i)
#define each(a, b) for(auto (a): (b))
#define all(v) (v).begin(),(v).end()
#define fi first
#define se second
#define pb push_back
#define show(x) cout <<#x<<" = "<<(x)<<endl
#define spair(p) cout <<#p<<": "<<p.fi<<" "<<p.se<<endl
#define svec(v) cout<<#v<<":";rep(kbrni,v.size())cout<<" "<<v[kbrni];cout<<endl
#define sset(s) cout<<#s<<":";each(kbrni,s)cout <<" "<<kbrni;cout<<endl

using namespace std;

typedef pair<int,int>P;

const int MAX_N = 2005;

ll dp[2][MAX_N][MAX_N];

int main()
{
    int n;
    cin >> n;
    vector<ll> a(n);
    rep(i,n){
        cin >> a[i];
    }
    rep(i,2){
        rep(j,n+1){
            rep(k,n+1){
                dp[i][j][k] = (1LL << 60);
            }
        }
    }
    dp[0][1][n] = a[0];
    dp[1][0][n-1] = a[n-1];
    for(int i=n-1;i>=0;--i){
        rep(j,n-i+1){
            int r = i + j;
            if(j >= 1){
                dp[0][j][r] = min(dp[0][j][r],max(dp[0][j-1][r]+1,a[j-1]));
                dp[0][j][r] = min(dp[0][j][r],max(dp[1][j-1][r]+r-j+1,a[j-1]));
            }
            if(r < n){
                dp[1][j][r] = min(dp[1][j][r],max(dp[1][j][r+1]+1,a[r]));
                dp[1][j][r] = min(dp[1][j][r],max(dp[0][j][r+1]+r-j+1,a[r]));
            }
        }
    }
    ll ans = (1LL << 60);
    rep(i,2){
        rep(j,n){
            ans = min(ans,dp[i][j+1][j+1]);
        }
    }
    cout << ans << endl;
    return 0;
}