package contest170210; import java.io.ByteArrayInputStream; import java.io.IOException; import java.io.InputStream; import java.io.PrintWriter; import java.util.Arrays; import java.util.InputMismatchException; public class E { InputStream is; PrintWriter out; String INPUT = ""; long[][][] dp; void solve() { int n = ni(); int[] a = na(n); int maxa = 0; for(int i = 0;i < n;i++){ maxa = Math.max(maxa, a[i]); } dp = new long[2][n][n]; long low = maxa-1, high = maxa + 5000000; while(high-low > 1){ long h = high+low>>1; if(ok(h, a)){ high = h; }else{ low = h; } } out.println(high); } boolean ok(long h, int[] a) { int n = a.length; long I = -Long.MAX_VALUE / 2; for(int len = 1;len <= n;len++){ for(int i = 0;i+len-1 < n;i++){ int j = i+len-1; if(len == 1){ dp[0][i][j] = h; dp[1][i][j] = h; if(dp[0][i][j] < a[i]){ dp[0][i][j] = I; } if(dp[1][i][j] < a[i]){ dp[1][i][j] = I; } }else{ dp[0][i][j] = Math.max(dp[0][i+1][j]-1, dp[1][i+1][j] - (j-i)); dp[1][i][j] = Math.max(dp[0][i][j-1]-(j-i), dp[1][i][j-1]-1); if(dp[0][i][j] < a[i]){ dp[0][i][j] = I; } if(dp[1][i][j] < a[j]){ dp[1][i][j] = I; } } } } return dp[0][0][n-1] >= 0 || dp[1][0][n-1] >= 0; } void run() throws Exception { is = INPUT.isEmpty() ? System.in : new ByteArrayInputStream(INPUT.getBytes()); out = new PrintWriter(System.out); long s = System.currentTimeMillis(); solve(); out.flush(); if(!INPUT.isEmpty())tr(System.currentTimeMillis()-s+"ms"); // Thread t = new Thread(null, null, "~", Runtime.getRuntime().maxMemory()){ // @Override // public void run() { // long s = System.currentTimeMillis(); // solve(); // out.flush(); // if(!INPUT.isEmpty())tr(System.currentTimeMillis()-s+"ms"); // } // }; // t.start(); // t.join(); } public static void main(String[] args) throws Exception { new E().run(); } private byte[] inbuf = new byte[1024]; public int lenbuf = 0, ptrbuf = 0; private int readByte() { if(lenbuf == -1)throw new InputMismatchException(); if(ptrbuf >= lenbuf){ ptrbuf = 0; try { lenbuf = is.read(inbuf); } catch (IOException e) { throw new InputMismatchException(); } if(lenbuf <= 0)return -1; } return inbuf[ptrbuf++]; } private boolean isSpaceChar(int c) { return !(c >= 33 && c <= 126); } private int skip() { int b; while((b = readByte()) != -1 && isSpaceChar(b)); return b; } private double nd() { return Double.parseDouble(ns()); } private char nc() { return (char)skip(); } private String ns() { int b = skip(); StringBuilder sb = new StringBuilder(); while(!(isSpaceChar(b))){ // when nextLine, (isSpaceChar(b) && b != ' ') sb.appendCodePoint(b); b = readByte(); } return sb.toString(); } private char[] ns(int n) { char[] buf = new char[n]; int b = skip(), p = 0; while(p < n && !(isSpaceChar(b))){ buf[p++] = (char)b; b = readByte(); } return n == p ? buf : Arrays.copyOf(buf, p); } private int[] na(int n) { int[] a = new int[n]; for(int i = 0;i < n;i++)a[i] = ni(); return a; } private long[] nal(int n) { long[] a = new long[n]; for(int i = 0;i < n;i++)a[i] = nl(); return a; } private char[][] nm(int n, int m) { char[][] map = new char[n][]; for(int i = 0;i < n;i++)map[i] = ns(m); return map; } private int[][] nmi(int n, int m) { int[][] map = new int[n][]; for(int i = 0;i < n;i++)map[i] = na(m); return map; } private int ni() { return (int)nl(); } private long nl() { long num = 0; int b; boolean minus = false; while((b = readByte()) != -1 && !((b >= '0' && b <= '9') || b == '-')); if(b == '-'){ minus = true; b = readByte(); } while(true){ if(b >= '0' && b <= '9'){ num = num * 10 + (b - '0'); }else{ return minus ? -num : num; } b = readByte(); } } private static void tr(Object... o) { System.out.println(Arrays.deepToString(o)); } }