#include <iostream>
#include <algorithm>
using namespace std;
int n,a[2005],dp[2][2005][2005],ans=2000000000;
int main(void){
    cin>>n;
    for(int i=1;i<=n;i++)cin>>a[i];
    for(int i=0;i<2;i++)for(int j=0;j<=n+1;j++)for(int k=0;k<=n+1;k++)dp[i][j][k]=2000000000;
    dp[0][2][n]=a[1];
    dp[1][1][n-1]=a[n];
    for(int k=n-2;k>=0;k--){
        for(int i=1;i+k<=n;i++){
            int l=i,r=i+k;
            dp[0][l+1][r]=min(dp[0][l+1][r],max(min(dp[0][l][r]+1,dp[1][l][r]+r-l+1),a[l]));
            dp[1][l][r-1]=min(dp[1][l][r-1],max(min(dp[0][l][r]+r-l+1,dp[1][l][r]+1),a[r]));
        }
    }
    for(int i=0;i<=n;i++){
        ans=min(ans,min(dp[0][i+1][i],dp[1][i+1][i]));
    }
    cout<<ans<<endl;
}