#include <bits/stdc++.h>
using ll = long long;
template <typename T>
constexpr T INF = std::numeric_limits<T>::max() / 16;
int main()
{
    int N;
    std::cin >> N;
    std::vector<ll> A(N + 1);
    for (int i = 1; i <= N; i++) { std::cin >> A[i]; }
    std::vector<std::vector<ll>> l(N + 2, std::vector<ll>(N + 2, INF<ll>)), r(N + 2, std::vector<ll>(N + 2, INF<ll>));
    l[2][N] = A[1], r[1][N - 1] = A[N];
    for (int i = N - 2; i >= 0; i--) {
        for (int j = 1; j <= N - i; j++) {
            const int L = j, R = j + i;
            l[L + 1][R] = std::min(l[L + 1][R], std::max(l[L][R] + 1, A[L]));
            r[L][R - 1] = std::min(l[L][R - 1], std::max(l[L][R] + R - L + 1, A[R]));
            l[L + 1][R] = std::min(l[L + 1][R], std::max(r[L][R] + R - L + 1, A[L]));
            r[L][R - 1] = std::min(l[L][R - 1], std::max(r[L][R] + 1, A[R]));
        }
    }
    ll ans = INF<ll>;
    for (int i = 1; i <= N + 1; i++) { ans = std::min({ans, l[i][i - 1], r[i][i - 1]}); }
    std::cout << ans << std::endl;
    return 0;
}