#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)1e6;
	int sum_temp = 0;

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

	sum_temp = sum;
	for (int i = 0; i < N; i++) {  
		int num = 0,stk = 0;
		sum = sum_temp;
		if (A[i] - (i + 1) > 0)
			num += A[i] - (i + 1);
		else
			stk += abs(A[i] - (i + 1));
		sum -= A[i];
		for (int j = 0; j < i; j++) {
			if (A[j] - (j + 1) > 0)
				num += A[j] - (j + 1);
			else
				stk += abs(A[j] - (j + 1));
			sum -= A[j];
		}
		int k = 0;
		for (int j = i+1; j < N; j++) {
			if (A[j] - (i-k) > 0)
				num += A[j] - (i-k);
			else
				stk += abs(A[j] - (i-k));

			sum -= A[j];
			k++;
			if (k >= i)break;
		}

		num += sum;
		if (N / 2 + N % 2 < i + 1) {
			if (num < ((i - 1)*i / 2)+stk)
				continue;
		}
		if (num < 0)continue;
		else ans = min(num, ans);
	}
	cout << ans << endl;
	return	0;
}