#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;

ll solve(int N, int res) {
	while (1) {
		if (((N / 2 + 1)*(N / 2 + 2) / 2 + (N / 2)*(N / 2 + 1) / 2) <= sum)break;
		res++;
		N--;
	}

	for (int i = 0; i < N; i++) {
		if (i <= N / 2) {
			if (A[i] - (i + 1) >= 0)
				res += A[i] - (i + 1);
		}
		else {
			if (A[i] - (N - i) >= 0)
				res += A[i] - (N - i);
		}
	}
	return res;
}

int main()
{
	int N;  cin >> N;
	int ans;

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

	if (N % 2 != 0) {
		ans = solve(N,0);
	}
	else {
		ans = min(solve(N - 1, 1), solve(N+1,0));
	}
	cout << ans << endl;
	return	0;
}