//Dedicated to my love, ivaziva
#include <bits/stdc++.h>
using namespace std;
using pii = pair<int, int>;
using ll = int64_t;
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define dbg(x) cerr << #x << ": " << x << '\n';
constexpr int N = 1e5 + 20;
int32_t main() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
cout.tie(nullptr);
cerr.tie(nullptr);
int n;
cin >> n;
vector<int> a(n);
vector<ll> ps(n, 0), ss(n, 0);
for (int i = 0; i < n; i++) {
cin >> a[i];
}
for (int i = 1; i < n; i++) {
ps[i] = ps[i - 1];
if (a[i] <= a[i - 1]) {
ps[i] += (a[i - 1] - a[i] + 1);
}
}
for (int i = n - 2; i >= 0; i--) {
ss[i] = ss[i + 1];
if (a[i] <= a[i + 1]) {
ss[i] += (a[i + 1] - a[i] + 1);
}
}
ll ans = 1e18;
for (int i = 0; i < n; i++) {
ps[i] = max(ps[i], ss[i]);
ans = min(ans, ps[i]);
}
cout << ans << '\n';
}
