#include<cstdio>
#include<iostream>
#include<algorithm>
#include<string>
#include<queue>
#include<vector>
#include<functional>
#include<cmath>
#include<map>
#include<stack>
#include<set>
#include<numeric>
#include<limits>

#define all(x) (x).begin(),(x).end()
#define rall(x) (x).rbegin(),(x).rend()

using namespace std;
typedef long long ll;
typedef pair<int, int> pi;
typedef pair<ll, ll> pl;
typedef pair<ll, string> pls;

int A[110],sum;

int main()
{
	int N;  cin >> N;
	int ans = (int)1e9;
	int sum_temp = 0;

	for (int i = 0; i < N; i++) {
		cin >> A[i];
		sum += A[i];
	}

	for (int i = 1; i <= 200; i += 2) {
		int not_enough = 0, remainder = 0;
		int aim = 0;

		for (int l = 0; l < i / 2 + 1; ++l) {
			aim++;
			if (l < N) {
				if (A[l] < aim) {
					not_enough += aim - A[l];
				}
				else {
					remainder += A[l] - aim;
				}
			}
			else {
				not_enough += aim;
			}
		}

		aim = 0;
		for (int r = i - 1; r >= i / 2 + 1; --r) {
			aim++;
			if (r < N) {
				if (A[r] < aim) {
					not_enough += aim - A[r];
				}
				else {
					remainder += A[r] - aim;
				}
			}
			else {
				not_enough += aim;
			}
		}
		for (int j = i; j < N; j++)
			remainder += A[j];

		if (remainder >= not_enough)
			ans = min(ans, remainder);
	}
	cout << ans << endl;
	return	0;
}