import std.conv, std.stdio, std.string;
import std.algorithm, std.array, std.bigint, std.container, std.math, std.numeric, std.range, std.regex, std.typecons;
import core.bitop;

class EOFException : Throwable { this() { super("EOF"); } }
string[] tokens;
string readToken() { for (; tokens.empty; ) { if (stdin.eof) { throw new EOFException; } tokens = readln.split; } auto token = tokens.front; tokens.popFront; return token; }
int readInt() { return readToken.to!int; }
long readLong() { return readToken.to!long; }
real readReal() { return readToken.to!real; }

void chmin(T)(ref T t, in T f) { if (t > f) t = f; }
void chmax(T)(ref T t, in T f) { if (t < f) t = f; }

int binarySearch(T)(in T[] as, in bool delegate(T) test) { int low = -1, upp = cast(int)(as.length); for (; low + 1 < upp; ) { int mid = (low + upp) >> 1; (test(as[mid]) ? low : upp) = mid; } return upp; }
int lowerBound(T)(in T[] as, in T val) { return as.binarySearch((T a) => (a < val)); }
int upperBound(T)(in T[] as, in T val) { return as.binarySearch((T a) => (a <= val)); }


enum INF = 10L^^18;

int N;
long[] A;

void main() {
  try {
    for (; ; ) {
      N = readInt();
      A = new long[N];
      foreach (i; 0 .. N) {
        A[i] = readLong();
      }
      
      auto dp = new long[][][](N, N, 2);
      foreach (i; 0 .. N) foreach (j; 0 .. N) {
        dp[i][j][] = INF;
      }
      dp[0][N - 1][] = 0;
      foreach_reverse (k; 1 .. N) {
        foreach (i; 0 .. N - k) {
          const j = i + k;
          foreach (s; 0 .. 2) {
            chmin(dp[i + 1][j][0], max(dp[i][j][s] + ((s == 0) ? 0 : (j - i)), A[i]) + 1);
            chmin(dp[i][j - 1][1], max(dp[i][j][s] + ((s == 1) ? 0 : (j - i)), A[j]) + 1);
          }
        }
      }
debug{
writeln(dp);
}
      long ans = INF;
      foreach (i; 0 .. N) {
        foreach (s; 0 .. 2) {
          chmin(ans, max(dp[i][i][s], A[i]));
        }
      }
      writeln(ans);
    }
  } catch (EOFException e) {
  }
}