#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]);
		vector<vi> dpL(N, vi(N, INF)), dpR = dpL;
		dpL[0][N - 1] = A[0];
		dpR[0][N - 1] = A[N - 1];
		for (int len = N - 1; len >= 1; -- len) {
			rer(i, 0, N - len) {
				int j = i + len - 1;
				int xL = INF, xR = INF;
				if (i > 0) {
					amin(xL, dpL[i - 1][j] + 1);
					amin(xR, dpL[i - 1][j] + (j - i + 1));
				}
				if (j < N - 1) {
					amin(xL, dpR[i][j + 1] + (j - i + 1));
					amin(xR, dpR[i][j + 1] + 1);
				}
				dpL[i][j] = max(A[i], xL);
				dpR[i][j] = max(A[j], xR);
			}
		}
		int ans = INF;
		rep(i, N)
			amin(ans, dpL[i][i]);
		printf("%d\n", ans);
	}
	return 0;
}