#define  _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <algorithm>
#include <utility>
#include <functional>
#include <cstring>
#include <queue>
#include <stack>
#include <math.h>
#include <iterator>
#include <vector>
#include <string>
#include <set>
#include <math.h>
#include <iostream> 
#include<map>
#include <iomanip>
#include <stdlib.h>
#include <list>
#include <typeinfo>
#include <list>
#include <set>
using namespace std;
#define MAX_MOD 1000000007
#define REP(i,n) for(long long i = 0;i < n;++i)
#define LONG_INF 800000000000000000
long long dp[300000] = {};
int main() {
#define int long long
	int n;
	cin >> n;
	vector<int> a;
	REP(i, n) {
		int tmp;
		cin >> tmp;
		a.push_back(tmp);
	}
	//左端に注目！
	int ans = LONG_INF;
	for (int i = 0;i < n+1;++i) {
		if (i != 0) {
			if(i != 1)
			dp[i] = max(dp[i - 1] + 1, a[i-1]);
			else dp[i] = a[i - 1];
		}
		else {
			dp[i] = 0;
		}
		//右端にGO
		if (i != n) {
			int pre_ans = dp[i] + (n - i - 1);
			for (int j = n - 1;j >= i;--j) {
				pre_ans = max(pre_ans + 1, a[j]);
			}
			ans = min(pre_ans, ans);
		}
	}
	ans = min(ans, dp[n]);
	cout << ans << endl;
	return 0;
}