#include <iostream>
#include <algorithm>
using namespace std;

#define N 100

int n;
int b[N * 2 + 1];
int t;

int main()
{
	cin >> n;
	for ( int i = 0; i < n; i++ ) {
		cin >> b[i];
		t += b[i];
	}

	int x = 0;
	int y = 0;
	while ( (x = x + y++ * 2 + 1) <= t ) {}
	y--;

	int z = 0;
	for ( int i = 0, j = (y - 1) * 2; i < y - 1; i++, j-- ) {
		z += max(0, b[i] - (i + 1));
		z += max(0, b[j] - (i + 1));
	}
	z += max(0, b[y - 1] - y);
	for ( int i = y * 2 - 1; i < n; i++ ) {
		z += b[i];
	}

	cout << z << endl;

	return 0;
}