import java.io.IOException; import java.util.*; import org.omg.CORBA.portable.InputStream; class Main { static int N; static int[] A; public static void main(String[] args) { Scanner sc = new Scanner(); N = sc.nextInt(); A = new int[N]; for (int i = 0; i < N; ++i) A[i] = sc.nextInt(); int[][][] dp = new int[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] = Integer.MAX_VALUE / 2; } } int ans = Integer.MAX_VALUE / 2; 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)); } static class Scanner { private final java.io.InputStream in = System.in; private final byte[] buffer = new byte[1024]; private int ptr = 0; private int buflen = 0; private boolean hasNextByte() { if (ptr < buflen) { return true; } else { ptr = 0; try { buflen = in.read(buffer); } catch (IOException e) { e.printStackTrace(); } if (buflen <= 0) { return false; } } return true; } private int readByte() { if (hasNextByte()) return buffer[ptr++]; else return -1; } private boolean isPrintableChar(int c) { return 33 <= c && c <= 126; } public boolean hasNext() { while (hasNextByte() && !isPrintableChar(buffer[ptr])) ptr++; return hasNextByte(); } public String next() { if (!hasNext()) throw new NoSuchElementException(); StringBuilder sb = new StringBuilder(); int b = readByte(); while (isPrintableChar(b)) { sb.appendCodePoint(b); b = readByte(); } return sb.toString(); } public long nextLong() { if (!hasNext()) throw new NoSuchElementException(); long n = 0; boolean minus = false; int b = readByte(); if (b == '-') { minus = true; b = readByte(); } if (b < '0' || '9' < b) { throw new NumberFormatException(); } while (true) { if ('0' <= b && b <= '9') { n *= 10; n += b - '0'; } else if (b == -1 || !isPrintableChar(b)) { return minus ? -n : n; } else { throw new NumberFormatException(); } b = readByte(); } } public int nextInt() { long nl = nextLong(); if (nl < Integer.MIN_VALUE || nl > Integer.MAX_VALUE) throw new NumberFormatException(); return (int) nl; } public double nextDouble() { return Double.parseDouble(next()); } } }