//#define _GLIBCXX_DEBUG
#include <bits/stdc++.h>
#define rep(i, n) for(int i=0; i<n; ++i)
#define all(v) v.begin(), v.end()
#define rall(v) v.rbegin(), v.rend()
using namespace std;
using ll = int64_t;
using ld = long double;
using P = pair<int, int>;
using vs = vector<string>;
using vi = vector<int>;
using vvi = vector<vi>;
template<class T> using PQ = priority_queue<T>;
template<class T> using PQG = priority_queue<T, vector<T>, greater<T> >;
const int INF = 0xccccccc;
const ll LINF = 0xcccccccccccccccLL;
template<typename T1, typename T2>
inline bool chmax(T1 &a, T2 b) {return a < b && (a = b, true);}
template<typename T1, typename T2>
inline bool chmin(T1 &a, T2 b) {return a > b && (a = b, true);}
template<typename T1, typename T2>
istream &operator>>(istream &is, pair<T1, T2> &p) { return is >> p.first >> p.second;}
template<typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2> &p) { return os << p.first << ' ' << p.second;}

#define N 2010

//head

int n;
ll a[N];
ll ans = LINF;
ll dp[N][N][2];

int main() {
	ios::sync_with_stdio(false);
	cin.tie(0);
	cin >> n;
	rep(i, n) cin >> a[i];
	fill(dp[0][0], dp[n][n+1], LINF);
	rep(i, 2) dp[0][n][i] = 0;
	for(int i = n; i > 0; i--) {
		rep(j, i) {
			chmin(dp[j+1][i][0], max(dp[j][i][0], a[j])+1);
			chmin(dp[j+1][i][1], max(dp[j][i][0], a[j])+i-j-1);
			chmin(dp[j][i-1][0], max(dp[j][i][1], a[i-1])+i-j-1);
			chmin(dp[j][i-1][1], max(dp[j][i][1], a[i-1])+1);
		}
	}
	//rep(i, n+1) for(int j = i; j <= n; j++) cout << dp[i][j][1] << (j == n?'\n':' ');
	rep(i, n+1) chmin(ans, min(dp[i][i][0], dp[i][i][1]));
	cout << ans << endl;
}