import java.util.*; class Main { static int N; static long[] A; public static void main(String[] args) { Scanner sc = new Scanner(System.in); N = sc.nextInt(); A = new long[N]; for (int i = 0; i < N; ++i) A[i] = Long.parseLong(sc.next()); long[][][] dp = new long[N][N][2]; for (int i = 0; i < N; ++i) { for (int j = 0; j < N; ++j) { for (int k = 0; k < 2; ++k) dp[i][j][k] = Long.MAX_VALUE / 10; } } long ans = Long.MAX_VALUE / 10; if (1 <= N - 1) { dp[1][N - 1][0] = A[0]; } else ans = Math.min(ans, A[0]); if (0 <= N - 2) { dp[0][N - 2][1] = A[N - 1]; } else { ans = Math.min(ans, A[N - 1]); } for (int len = N - 2; len >= 1; --len) { for (int left = 0; left < N; ++left) { int right = left + len - 1; if (right >= N) break; if (left > 0) { dp[left][right][0] = Math.min(dp[left - 1][right][1] + len + 1, dp[left - 1][right][0] + 1); dp[left][right][0] = Math.max(dp[left][right][0], A[left - 1]); } if (right + 1 < N) { dp[left][right][1] = Math.min(dp[left][right + 1][1] + 1, dp[left][right + 1][0] + len + 1); dp[left][right][1] = Math.max(dp[left][right][1], A[right + 1]); } } } for (int i = 0; i < N; ++i) { ans = Math.min(ans, Math.max(dp[i][i][0] + 1, A[i])); ans = Math.min(ans, Math.max(dp[i][i][1] + 1, A[i])); } System.out.println(ans); } static void tr(Object... objects) { System.out.println(Arrays.deepToString(objects)); } }