#include<iostream>
#include<vector>
#include<algorithm>

using namespace std;
using ll = long long;

int main(){
    cin.tie(nullptr);
    ios::sync_with_stdio(false);

    int n;
    cin>>n;
    vector<ll> a(n);
    for(int i = 0;i<n;i++) cin>>a[i];
    ll ans = 1e18;
    vector<vector<vector<ll>>> dp(n,vector<vector<ll>>(n,vector<ll>(2,1e18)));
    dp[0][n-1][0] = -1;
    
    for(int i = n;i>0;i--){
        for(int j = 0;j<n;j++){
            int le = j;
            int ri = le + i - 1;
            if(ri>=n) continue;
            for(int t = 0;t<2;t++){
                int p;
                if(t==0) p = le - 1;
                else p = ri + 1;
                for(int nt = 0;nt<2;nt++){
                    int np;
                    if(nt==0) np = le;
                    else np = ri;
                    ll ti = dp[le][ri][t];
                    ti += abs(p-np);
                    if(nt==0) ti = max(ti,a[le]);
                    else ti = max(ti,a[ri]);
                    if(nt==0){
                        if(ri==le){
                            ans = min(ans,ti);
                        }else{
                            dp[le+1][ri][nt] = min(dp[le+1][ri][nt],ti);
                        }
                    }else{
                        if(ri==le){
                            ans = min(ans,ti);
                        }else{
                            dp[le][ri-1][nt] = min(dp[le][ri-1][nt],ti);
                        }
                    }
                }
            }
        }
    }

    cout<<ans<<endl;
    
}