#include <bits/stdc++.h>
#define int long long
using namespace std;
int n;
int a[2005];
int f[2005][2005];
void read(){
    cin>>n;
    for(int i=1;i<=n;i++){
        cin>>a[i];
    }
}
void work(){
    memset(f,0x3f,sizeof(f));
    f[1][n]=f[n][1]=0;
    for(int l=n;l>1;l--){
        for(int i=1;i+l-1<=n;i++){
            int j=i+l-1;
            f[i][j]=min(f[i][j],f[j][i]+(j-i-1));
            f[j][i]=min(f[j][i],f[i][j]+(j-i-1));
            f[i+1][j]=min(f[i+1][j],max(f[i][j],a[i])+1);
            f[j-1][i]=min(f[j-1][i],max(f[j][i],a[j])+1);
        }
    }
    int ans=f[0][0];
    for(int i=1;i<=n;i++){
        ans=min(ans,max(f[i][i],a[i]));
    }
    cout<<ans<<endl;
}
signed main(){
    read();
    work();
    return 0;
}