#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
int n;
int dp[2000][2000][2];
ll a[2000],t[2000];
int dfs(int l,int r,int k){
	if (dp[l][r][k] != -1) return dp[l][r][k];
    if (k == 0)
    {
        int x = dfs(l + 1, r, 0) + 1;
        int y = dfs(l + 1, r, 1) + r - l;
        dp[l][r][k]=min(x,y);
        if (t[l] < dp[l][r][k]) dp[l][r][k] = 1000000000;
    }
    else
    {
        int x = dfs(l, r-1, 1) + 1;
        int y = dfs(l, r-1, 0) + r - l;
        dp[l][r][k]=min(x,y);
        if (t[r] < dp[l][r][k]) dp[l][r][k] = 1000000000;
    }
    return dp[l][r][k];
        	
}
int f(ll x){
	for(int i=0;i<n;i++){
		t[i]=x-a[i];
		if(t[i]<0)return 0;
	}
	for(int i=0;i<n;i++)
		for(int j=i;j<n;j++){
			if(i!=j)dp[i][j][0]=dp[i][j][1]=-1;
			else dp[i][j][0]=dp[i][j][1]=0;
		}
		if(min(dfs(0,n-1,0),dfs(0,n-1,1))<1000000000)return 1;
		else return 0;
}
int main(){
	cin>>n;
	for(int i=0;i<n;i++)cin>>a[i];
	ll l=0,r=10000000000;
	for(int iii=0;iii<40;iii++){
		ll m=(l+r)/2;
		if(f(m))r=m;
		else l=m;
	}
	cout<<r<<endl;
}