#include <iostream>
#include <algorithm>
using namespace std;
const int N = 200003;
int n;
int a[N];
int dl[N], dr[N];
long long fa = 1e18;
int main() {
cin >> n;
for (int i = 1; i <= n; i++)
{
cin >> a[i];
}
for (int k = 1; k <= n; k++)
{
for (int i = 1; i <= n; i++)
{
dl[i] = a[i] - a[i - 1];
dr[i] = a[i] - a[i + 1];
}
int l = 1, r = n;
long long ans = 0;
while (l != r)
{
//cout << l << ' ' << r << ' ' << ans << endl;
if (r == k)
{
int d = max(-dl[l] + 1, 0);
ans += d;
dl[l] += d;
dr[r] += d;
l++;
}
else if (l == k)
{
int d = max(-dr[r] + 1, 0);
ans += d;
dl[l] += d;
dr[r] += d;
r--;
}
else if (dl[l] > dr[r])
{
int d = max(-dl[l] + 1, 0);
ans += d;
dl[l] += d;
dr[r] += d;
l++;
}
else
{
int d = max(-dr[r] + 1, 0);
ans += d;
dl[l] += d;
dr[r] += d;
r--;
}
}
ans += max(max(-dl[l] + 1, 0), max(-dr[r] + 1, 0));
//cout << ans << endl;
fa = min(ans, fa);
}
cout << fa;
return 0;
}
