ABC 336

void solve() {
    ll N;
    cin >> N;
    vll A(N);
    rep(i, N) cin >> A[i];

    A.push_back(0);
    A.insert(A.begin(), 0);

    N += 2;
    vll dl(N, 0), dr(N, 0);
    rep2(i, 1, N) {
        dl[i] = min(dl[i - 1] + 1, A[i]);
    }

    rep(i, N - 1) {
        int k = N - 1 - i;
        dr[k - 1] = min(dr[k] + 1, A[k - 1]);
    }

    ll ans = 0;
    rep(i, N) {
        chmax(ans, min(dr[i], dl[i]));
    }
    cout << ans << endl;
}