#include<bits/stdc++.h>
using namespace std;
#define rep(i,a,b) for(int i=a;i<b;i++)
#define rrep(i,a,b) for(int i=a;i>=b;i--)



int N;
int A[2010];
//-----------------------------------------------------------------
int sol(int i) {
	int t = -1;
	rep(j, 0, N) {
		if (j < i) {
			t = max(t + 1, A[j]);
		} else {
			t++;
		}
	}

	rrep(j, N - 1, i) {
		t = max(t + 1, A[j]);
	}

	int s = -1;
	rep(j, 0, N) s = max(s + 1, A[j]);
	t = min(s, t);

	return t;
}
//-----------------------------------------------------------------
int main() {
	cin >> N;
	rep(i, 0, N) scanf("%d", &A[i]);

	int ans = INT_MAX / 2;
	rep(i, 0, N) ans = min(ans, sol(i));
	reverse(A, A + N);
	rep(i, 0, N) ans = min(ans, sol(i));
	cout << ans << endl;
}