#include <iostream>
#include <vector>
#include <algorithm>
#include <cstdlib>
#include <limits>
#include <numeric>

using namespace std;

int main() {
	int n;
	cin >> n;
	vector<int> a(n);
	for (int i = 0; i < n; ++i) {
		cin >> a[i];
	}
	int givenBlocks = accumulate(a.begin(), a.end(), 0);
	int minMovement = numeric_limits<int>::max();
	for (int height = 1;; ++height) {
		int width = height * 2 - 1;
		int blocks = 0;
		int movement = 0;
		for (int x = 0; x < max<int>(width, a.size()); ++x) {
			int y;
			if (x < height) {
				y = x + 1;
			} else if (x < width) {
				y = 2 * height - x - 1;
			} else {
				y = 0;
			}
			blocks += y;
			int current = ((size_t)x < a.size() ? a[x] : 0);
			if (y < current) {
				movement += current - y;
			}
		}
		if (givenBlocks < blocks) {
			break;
		}
		minMovement = min(minMovement, movement);
	}
	cout << minMovement << endl;
	return 0;
}