#include <bits/stdc++.h>
using namespace std;

#define fi first
#define se second
#define repl(i,a,b) for(int i=(int)(a);i<(int)(b);i++)
#define rep(i,n) repl(i,0,n)
#define each(itr,v) for(auto itr:v)
#define pb(s) push_back(s)
#define mp(a,b) make_pair(a,b)
#define all(x) (x).begin(),(x).end()
#define dbg(x) cout<<#x"="<<x<<endl
#define maxch(x,y) x=max(x,y)
#define minch(x,y) x=min(x,y)
#define uni(x) x.erase(unique(all(x)),x.end())
#define exist(x,y) (find(all(x),y)!=x.end())
#define bcnt(x) bitset<32>(x).count()

typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> P;
typedef pair<P, int> PPI;
typedef pair<ll, ll> PL;
typedef pair<P, ll> PPL;

#define INF 1e15

#define MAX_N 1000

ll n;
ll a[2222];

ll dp[2222][2222][2];

int main(){
	cin.sync_with_stdio(false);
	cin>>n;
	rep(i,n)cin>>a[i];
	rep(i,2222)rep(j,2222)rep(k,2)dp[i][j][k]=INF;
	dp[0][n-1][0]=a[0];
	dp[0][n-1][1]=a[n-1];
	for(int len=n-1;len>0;len--)rep(l,n-len){
		int r=l+len;
		minch(dp[l+1][r][0],max(dp[l][r][0]+1,a[l+1]));
		minch(dp[l+1][r][1],max(dp[l][r][0]+len,a[r]));
		minch(dp[l][r-1][0],max(dp[l][r][1]+len,a[l]));
		minch(dp[l][r-1][1],max(dp[l][r][1]+1,a[r-1]));
	}
	ll res=INF;
	rep(i,n)minch(res,min(dp[i][i][0],dp[i][i][1]));
	cout<<res<<endl;
	return 0;
}