#define _USE_MATH_DEFINES
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;
typedef vector<int> vei;
typedef vector<vei> vevei;
#define all(a) (a).begin(), (a).end()
#define sz(a) (int) a.size()
#define con cout << "NO\n"
#define coe cout << "YES\n";
#define str string
#define pb push_back
#define ff first
#define sc second
#define ss second
#define pii pair<int, int>
#define mxe max_element
#define mne min_element
#define stf shrink_to_fit
#define f(i, l, r) for (int i = (l); i < (r); i++)
#define double ld
#define int ll
signed main() {
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
cout.tie(nullptr);
int n; cin >> n;
vector<int> a(n);
f(i, 0, n) cin >> a[i];
vector<int> pref(n + 1, 0);
for (int i = 2; i <= n; i++) {
pref[i] = pref[i - 1] + (a[i - 1] > a[i - 2] ? 0 : a[i - 2] - a[i - 1] + 1);
}
vector<int> suff(n + 1, 0);
for (int i = 2; i <= n; i++) {
suff[i] = suff[i - 1] + (a[n - i] > a[n - i + 1] ? 0 : a[n - i + 1] - a[n - i] + 1);
}
int ans = 1e18;
f(i, 0, n) {
ans = min(ans, max(pref[i + 1], suff[n - i]));
}
cout << ans << '\n';
return 0;
}
