#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef double db;
#define int ll
#define all(x) (x).begin(), (x).end()
#define md ((tl + tr) >> 1)
#define TL v + v, tl, md
#define TR v + v + 1, md + 1, tr
#define F first
#define S second
#define pii pair<int, int>
const int maxn = 5e5 + 7;
const int maxw = 1e6 + 7;
const ll inf = 1e18 + 7;
const double PI = atan2(0.0, -1.0);
int n, a[maxn], pref[maxn], suf[maxn];
void solve() {
cin >> n;
for (int i = 1; i <= n; i++)
cin >> a[i];
for (int i = 2; i <= n; i++) {
pref[i] = pref[i - 1] + max(a[i], a[i - 1] + 1) - a[i];
}
for (int i = n - 1; i >= 1; i--) {
suf[i] = suf[i + 1] + max(a[i], a[i + 1] + 1) - a[i];
}
int ans = inf;
for (int i = 1; i <= n; i++)
ans = min(ans, max(pref[i], suf[i]));
cout << ans;
}
signed main() {
ios_base::sync_with_stdio(false);
cin.tie(0);
int test = 1;
// cin >> test;
while (test--) {
solve();
// cout << '\n';
}
return 0;
}
