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

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 ll = 0;
		long long rr = 0;
		long long tt = 0;
		for (int i = 0; i < n; i++) {
			long long y = a[i];
			long long x = i - ii;
			long long yy = y + x;
			long long xx = y - x;
			ll = min(ll, xx);
			rr = max(rr, xx);
			tt = max(tt, yy);
		}
		for (int jj = 0; jj < 2; jj++) {
			if (rr % 2 == 0) {
				ans = min(ans, rr / 2 + (tt + 1) / 2 + (rr - ll + 1) / 2);
			} else {
				ans = min(ans, rr / 2 + tt / 2 + (rr - ll + 1) / 2);
			}
			swap(ll, rr);
			ll *= -1;
			rr *= -1;
		}
	}
	cout << ans << endl;
}