#include <bits/stdc++.h>
#define int long long
using namespace std;
signed main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
int n;
cin >> n;
vector<int> a(n + 1), pref(n + 1, 0), suff(n + 1, 0);
for (int i = 1; i <= n; i++) {
cin >> a[i];
}
for (int i = 2; i <= n; i++) {
if (a[i] < a[i - 1] + 1) {
pref[i] = pref[i - 1] + a[i - 1] + 1 - a[i];
}
else {
pref[i] = pref[i - 1];
}
}
for (int i = n - 1; i >= 1; i--) {
if (a[i] < a[i + 1] + 1) {
suff[i] = suff[i + 1] + a[i + 1] + 1 - a[i];
}
else {
suff[i] = suff[i + 1];
}
}
int ans = 1e18;
for (int i = 1; i <= n; i++) {
ans = min(ans, max(pref[i], suff[i]));
}
cout << ans;
}
