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)); }


int N;
long[] A;

long[][][] dp;

bool check(long tl) {
  foreach (i; 0 .. N) foreach (j; 0 .. N) {
    dp[i][j][] = -1;
  }
  foreach (i; 0 .. N) {
    dp[i][i][0] = tl;
    dp[i][i][1] = tl;
  }
  foreach (k; 0 .. N) {
    foreach (i; 0 .. N - k) {
      const j = i + k;
      foreach (s; 0 .. 2) {
        if (0 <= i - 1) {
          const c = dp[i][j][s] - abs((i - 1) - (s ? j : i));
          if (c >= A[i - 1]) {
            chmax(dp[i - 1][j][0], c);
          }
        }
        if (j + 1 < N) {
          const c = dp[i][j][s] - abs((j + 1) - (s ? j : i));
          if (c >= A[j + 1]) {
            chmax(dp[i][j + 1][1], c);
          }
        }
      }
    }
  }
debug{
writeln("tl = ",tl);
writeln("  dp = ",dp);
}
  return (dp[0][N - 1].maxElement >= 0);
}

void main() {
  try {
    for (; ; ) {
      N = readInt();
      A = new long[N];
      foreach (i; 0 .. N) {
        A[i] = readLong();
      }
      
      dp = new long[][][](N, N, 2);
      
      long lo = A.maxElement - 1, hi = A.maxElement + N;
      for (; lo + 1 < hi; ) {
        const mid = (lo + hi) / 2;
        (check(mid) ? hi : lo) = mid;
      }
      writeln(hi);
    }
  } catch (EOFException e) {
  }
}