#include <iostream>
#define INT_MAX 2147483647
using namespace std;
int a[200005];
int cr[200005], cn[200005];
int dr[200005], dn[200005];
int main()
{
int n;
cin >> n;
for (int i = 0; i < n; i++)
{
cin >> a[i];
}
cn[0] = a[0];
for (int i = 1; i < n; i++)
{
cn[i] = a[i] + cr[i - 1];
cr[i] = cr[i - 1];
if (cn[i] <= cn[i - 1])
{
cr[i] += (cn[i - 1] - cn[i]) + 1;
cn[i] = cn[i - 1] + 1;
}
}
dn[n - 1] = a[n - 1];
for (int i = n - 2; i >= 0; i--)
{
dn[i] = a[i] + dr[i + 1];
dr[i] = dr[i + 1];
if (dn[i] <= dn[i + 1])
{
dr[i] += (dn[i + 1] - dn[i]) + 1;
dn[i] = dn[i + 1] + 1;
}
}
int r = INT_MAX;
for (int i = 0; i < n; i++)
{
r = min(r, max(cr[i], dr[i]));
}
cout << r;
}
