#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 pl[N], pr[N];
ll ppl[N], spr[N];
ll hp[N], hs[N];
void solv()
{
cin >> n;
for (int i = 1; i <= n; ++i)
cin >> a[i];
for (int i = 1; i <= n; ++i)
{
if (a[i - 1] + pl[i - 1] < a[i])
{
pl[i] = 0;
}
else
{
pl[i] = (a[i - 1] + pl[i - 1] + 1 - a[i]);
}
}
for (int i = n; i >= 1; --i)
{
if (a[i + 1] + pr[i + 1] < a[i])
{
pr[i] = 0;
}
else
{
pr[i] = (a[i + 1] + pr[i + 1] + 1 - a[i]);
}
}
for (int i = 1; i <= n; ++i)
{
ppl[i] = ppl[i - 1] + pl[i];
}
for (int i = n; i >= 1; --i)
{
spr[i] = spr[i + 1] + pr[i];
}
for (int i = 1; i <= n; ++i)
{
hp[i] = hp[i - 1] + min(pl[i], pl[i - 1]);
}
for (int i = n; i >= 1; --i)
{
hs[i] = hs[i + 1] + min(pr[i], pr[i + 1]);
}
ll ans = INF;
for (int i = 1; i <= n; ++i)
{
ll yans = ppl[i - 1] + spr[i + 1];
yans -= hp[i - 1];
yans -= hs[i + 1];
ll pp;
if (a[i] > max(a[i - 1] + pl[i - 1], a[i + 1] + pr[i + 1]))
pp = 0;
else
pp = max(a[i - 1] + pl[i - 1], a[i + 1] + pr[i + 1]) + 1 - a[i];
yans += pp;
yans -= min(pl[i - 1], pp);
yans -= min(pr[i + 1], pp);
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;
}
