#include "bits/stdc++.h"
using namespace std;
#define rep(i,n) for(int (i)=0;(i)<(int)(n);++(i))
#define rer(i,l,u) for(int (i)=(int)(l);(i)<=(int)(u);++(i))
#define reu(i,l,u) for(int (i)=(int)(l);(i)<(int)(u);++(i))
static const int INF = 0x3f3f3f3f; static const long long INFL = 0x3f3f3f3f3f3f3f3fLL;
typedef vector<int> vi; typedef pair<int, int> pii; typedef vector<pair<int, int> > vpii; typedef long long ll;
template<typename T, typename U> static void amin(T &x, U y) { if (y < x) x = y; }
template<typename T, typename U> static void amax(T &x, U y) { if (x < y) x = y; }

int main() {
	int N;
	while (~scanf("%d", &N)) {
		vector<int> A(N);
		for (int i = 0; i < N; ++ i)
			scanf("%d", &A[i]);
		int lo = *max_element(A.begin(), A.end()), up = lo + N - 1;
		while (up - lo > 0) {
			int mid = (lo + up) / 2;
			vector<vi> dpL(N, vi(N, INF)), dpR = dpL;
			rep(i, N) if(A[i] <= mid)
				dpL[i][i] = 0;
			rer(len, 1, N - 1) {
				rer(i, 0, N - len) {
					int j = i + len - 1;
					int xL = dpL[i][j], xR = dpR[i][j];
					if (xL == INF && xR == INF) continue;
					if (i > 0) {
						int t = min(xL + 1, xR + (j - i + 1));
						if(t <= mid - A[i - 1])
							amin(dpL[i - 1][j], t);
					}
					if (j + 1 < N) {
						int t = min(xL + (j - i + 1), xR + 1);
						if (t <= mid - A[j + 1])
							amin(dpR[i][j + 1], t);
					}
				}
			}
			if (min(dpL[0][N - 1], dpR[0][N - 1]) <= mid)
				up = mid;
			else
				lo = mid + 1;
		}
		int ans = lo;
		printf("%d\n", ans);
	}
	return 0;
}