#include <bits/stdc++.h>
using namespace std;
#define m_p make_pair
#define all(x) (x).begin(),(x).end()
#define sz(x) ((int)(x).size())
#define fi first
#define se second
typedef long long ll;
mt19937 rnd(chrono::steady_clock::now().time_since_epoch().count());
mt19937 rnf(2106);
const int N = 200005;
const ll INF = 1000000009000000009;
int n;
int a[N];
ll p[N], s[N];
void solv()
{
cin >> n;
for (int i = 1; i <= n; ++i)
cin >> a[i];
for (int i = 1; i <= n; ++i)
{
p[i] = p[i - 1];
if (a[i] <= a[i - 1])
p[i] += (a[i - 1] + 1 - a[i]);
}
for (int i = n; i >= 1; --i)
{
s[i] = s[i + 1];
if (a[i] <= a[i + 1])
s[i] += (a[i + 1] + 1 - a[i]);
}
ll ans = INF;
for (int i = 1; i <= n; ++i)
{
ll x = a[i - 1], y = a[i], z = a[i + 1];
if (p[i - 1] > s[i + 1])
{
x += p[i - 1] - s[i + 1];
y += p[i - 1] - s[i + 1];
}
else
{
y += s[i + 1] - p[i - 1];
z += s[i + 1] - p[i - 1];
}
ll yans = max(p[i - 1], s[i + 1]);
if (y <= max(x, z))
yans += max(x, z) + 1 - y;
ans = min(ans, yans);
}
cout << ans << "\n";
}
int main()
{
#ifdef SOMETHING
freopen("input.txt", "r", stdin);
//freopen("output.txt", "w", stdout);
#endif // SOMETHING
ios_base::sync_with_stdio(false), cin.tie(0);
int tt = 1;
//cin >> tt;
while (tt--)
{
solv();
}
return 0;
}
