#include<bits/stdc++.h>
typedef long long ll;
typedef unsigned long long ull;
using namespace std;
#define pb push_back
int dy[]={0, 0, 1, -1, 1, 1, -1, -1};
int dx[]={1, -1, 0, 0, 1, -1, -1, 1};

#define FOR(i,a,b) for (int i=(a);i<(b);i++)
#define RFOR(i,a,b) for (int i=(b)-1;i>=(a);i--)
#define REP(i,n) for (int i=0;i<(n);i++)
#define RREP(i,n) for (int i=(n)-1;i>=0;i--)
#define mp make_pair
#define fi first
#define sc second
ll INF = 1e12;
ll dp[3000][3000][2];
ll n,a[3000];
int main(){
	cin >> n;

	REP(i,n) {
		cin >> a[i];
		REP(j,n) {
			dp[i][j][0] = dp[i][j][1] = INF;
		}
	}

	dp[0][n - 1][true] = a[n - 1];
	dp[0][n - 1][false] = a[0];

	RFOR(k,1,n) {
		for(int i = 0;i + k < n;i++) {
			int j = i + k;

			dp[i + 1][j][false] = min(dp[i + 1][j][false],max(dp[i][j][false] + 1,a[i + 1]));
			dp[i + 1][j][true] = min(dp[i + 1][j][true],max(dp[i][j][false] + j - i,a[j]));
			dp[i][j - 1][false] = min(dp[i][j - 1][false],max(dp[i][j][true] + j - i,a[i]));
			dp[i][j - 1][true] = min(dp[i][j - 1][true],max(dp[i][j][true] + 1,a[j - 1]));
		}
	}

	/*RFOR(k,1,n) {
		REP(i,k + 1) {
			int j = k - 1;
			if(i > j)
				continue;
			cout << dp[i][j][true] << " " << dp[i][j][false] << "   ";
		}
		cout << endl;
	}*/

	ll ans = INF;
	REP(i,n) {
		ans = min(ans,dp[i][i][true]);
	}
	cout << ans << endl;
	return 0;
}