#include <iostream>
#include <vector>
#include <algorithm>
#include <set>

using namespace std;

int main() {
	int n;
	cin >> n;
	vector<long long> a(n);
	for (int i = 0; i < n; i++) {
		cin >> a[i];
	}

	long long ans = 1e18;
	for (int ii = 0; ii < n; ii++) {
		long long rr = 0;
		long long tt = 0;
		set<pair<long long, long long>> st;
		for (int i = 0; i < n; i++) {
			long long y = a[i];
			long long x = i - ii;
			y += max(0LL, x - y);
			long long yy = y + x;
			long long xx = y - x;
			rr = max(rr, xx);
			tt = max(tt, yy);
			st.emplace(xx, yy);
		}
		if (st.count(make_pair(rr, tt))) {
			ans = min(ans, (tt + 1) / 2 + (rr + 1) / 2);
		} else {
			ans = min(ans, (tt + rr + 1) / 2);
		}
	}
	cout << ans << endl;
}