#include <iostream>
using namespace std;
#define FOR(i,s,e) for(int i = (s);i <=(e);i++)
int total;
int N;
int a[100];

int result = 10000;

int main()
{
	cin >> N;

	FOR(i,0,N - 1)
	{
		cin >> a[i];
		total += a[i];
	}

	int memo = 0;

	FOR(i,1,N)
	{
		memo += i;

		if(total < memo || i > N / 2 + 1) break;
		int cou = 1;
		int temp = 0;
		FOR(j,0,i - 1)
		{
			if(cou < a[j]) temp += a[j] - cou;
			cou++;
		}
		cou = i - 1;
		FOR(j,i,i + i - 1)
		{
			if(cou < a[j]) temp += a[j] - cou;
			cou--;
		}

		FOR(j,i + i,N - 1)
		{
			temp += a[j];
		}
		result = min(result,temp);
	}

	cout << result << endl;
	return 0;
}