#define _USE_MATH_DEFINES
#include <cstdio>
#include <iostream>
#include <sstream>
#include <fstream>
#include <iomanip>
#include <algorithm>
#include <cmath>
#include <complex>
#include <string>
#include <vector>
#include <list>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <bitset>
#include <numeric>
#include <limits>
#include <climits>
#include <cfloat>
#include <functional>
#include <iterator>
using namespace std;

const int INF = INT_MAX / 2;

int main()
{
    int n;
    cin >> n;
    vector<int> a(n);
    for(int i=0; i<n; ++i)
        cin >> a[i];

    vector<vector<vector<int> > > dp(n, vector<vector<int> >(n, vector<int>(2, INF)));
    dp[0][n-1][0] = 0;
    dp[0][n-1][1] = 1;
    for(int len=n-1; len>0; --len){
        for(int i=0; i<n-len; ++i){
            int j = i + len;
            for(int k=0; k<2; ++k){
                int curr = (k == 0) ? i : j;
                int t1 = max(a[i], dp[i][j][k] + abs(curr - i)) + 1;
                dp[i+1][j][k] = min(dp[i+1][j][k], t1);
                int t2 = max(a[j], dp[i][j][k] + abs(curr - j)) + 1;
                dp[i][j-1][k] = min(dp[i][j-1][k], t2);
            }
        }
    }

    int ans = INF;
    for(int i=0; i<n; ++i){
        for(int j=0; j<2; ++j){
            ans = min(ans, max(a[i], dp[i][i][j]));
        }
    }
    cout << ans << endl;

    return 0;
}