#include <iostream>
#include <vector>
#include <algorithm>
#define rep(i,n) for(int i = 0; (i) < int(n); ++(i))
#define rep_rev(i,n) for(int i = (n)-1;(i) >= 0; --(i))
using namespace std;

template<typename X,typename T> auto vectors(X x, T a){return vector<T>(x,a);}
template<typename X,typename Y, typename Z,typename... Zs> auto vectors(X x,Y y,Z z,Zs... zs){
    auto cont = vectors(y, z, zs...); return vector<decltype(cont)>(x, cont);
}
const int inf = 1.01e9;

int main(){
    int n; cin >> n;
    vector<int> a(n); rep(i,n) cin >> a[i];
    auto dp = vectors(2,n+1,n+1,inf);

    dp[false][1][n] = a[0];
    dp[true][0][n-1] = a[n-1];

    rep_rev(len,n+1){
        rep(l,n - len + 1){
            int r = l + len;
            if(0 <= l-1){
                dp[false][l][r] = min(dp[false][l][r],max(dp[false][l-1][r]+1,    a[l-1]));
                dp[false][l][r] = min(dp[false][l][r],max(dp[true ][l-1][r]+r-l+1,a[l-1]));
            }
            if(r+1 < n+1){
                dp[true][l][r] = min(dp[true][l][r],max(dp[false][l][r+1]+r-l+1,a[r]));
                dp[true][l][r] = min(dp[true][l][r],max(dp[true ][l][r+1]+1,     a[r]));
            }
        }
    }
    int ans = inf;
    rep(p,2) rep(l,n+1) ans = min(ans,dp[p][l][l]);

    cout << ans << endl;
    return 0;
}