/*
* * author: attacker
* * created: 04.02.2026 18:39:36
*/
#include <bits/stdc++.h>
using namespace std;
#ifdef LOCAL
#include "algo/debug.h"
#else
#define debug(...) 42
#endif
#define mt_rng mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
int main() {
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
cout.tie(nullptr);
int n;
cin >> n;
vector<int64_t> a(n);
for (int i = 0; i < n; i++) {
cin >> a[i];
}
vector<int64_t> pref(n);
pref[0] = 0;
for (int i = 1; i < n; i++) {
if (a[i] <= a[i - 1]) {
pref[i] = pref[i - 1] + a[i - 1] + 1 - a[i];
} else {
pref[i] = pref[i - 1];
}
}
vector<int64_t> suff(n);
suff[n - 1] = 0;
for (int i = n - 2; i >= 0; i--) {
if (a[i] <= a[i + 1]) {
suff[i] = suff[i + 1] + a[i + 1] + 1 - a[i];
} else {
suff[i] = suff[i + 1];
}
}
int64_t res = int64_t(1e18);
for (int i = 0; i < n; i++) {
res = min(res, max(pref[i], suff[i]));
}
cout << res << '\n';
return 0;
}
