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) { } }