n = int(input()) A = list(map(int, input().split())) # Convert to 1-based index A = [0] + A INF = 10**18 # dp[l][r][0] represents time when at left end l after covering [l..r] # dp[l][r][1] represents time when at right end r after covering [l..r] dp = [[[INF] * 2 for _ in range(n + 2)] for __ in range(n + 2)] for i in range(1, n + 1): dp[i][i][0] = A[i] dp[i][i][1] = A[i] for length in range(2, n + 1): for l in range(1, n - length + 2): r = l + length - 1 # Update dp[l][r][0]: come from [l+1][r] which is left end (l+1) time_candidate = dp[l+1][r][0] + 1 new_time = max(time_candidate, A[l]) if new_time < dp[l][r][0]: dp[l][r][0] = new_time # Update dp[l][r][1]: come from [l][r-1] which is right end (r-1) time_candidate = dp[l][r-1][1] + 1 new_time = max(time_candidate, A[r]) if new_time < dp[l][r][1]: dp[l][r][1] = new_time result = min(dp[1][n][0], dp[1][n][1]) print(result)