#include<bits/stdc++.h>
using namespace std;
mt19937 rnd(51);
#define ll long long
#define pb push_back
#define ld long double
const ll INF = 1e18;
signed main() {
#ifdef LOCAL
freopen("input.txt", "r", stdin);
#endif // LOCAL
ios_base::sync_with_stdio(0);
cin.tie(0);
int n;
cin >> n;
vector<int> a(n);
vector<ll> pr(n), sf(n);
for (int i = 0; i < n; i++) {
cin >> a[i];
}
for (int i = 1; i < n; i++) {
pr[i] = pr[i - 1];
if (a[i - 1] >= a[i]) {
pr[i] += a[i - 1] - a[i] + 1;
}
}
for (int i = n - 2; i >= 0; i--) {
sf[i] = sf[i + 1];
if (a[i] <= a[i + 1]) {
sf[i] += a[i + 1] - a[i] + 1;
}
}
ll res = INF;
for (int i = 0; i < n; i++) {
ll cnt0 = (i > 0 ? pr[i - 1] : 0), cnt1 = (i + 1 < n ? sf[i + 1] : 0);
ll need = -1;
if (i > 0) {
need = max(need, a[i - 1] + cnt0 + 1);
}
if (i + 1 < n) {
need = max(need, a[i + 1] + cnt1 + 1);
}
res = min(res, max(0ll, need - a[i] - max(cnt0, cnt1)) + max(cnt0, cnt1));
}
cout << res << endl;
return 0;
}
