#include <bits/stdc++.h>
#define fr first
#define sc second
#define pii pair<int, int>
#define pb push_back
#define szof(s) (int)s.size()
#define all(s) s.begin(), s.end()
#define fastInp ios_base::sync_with_stdio(0); cin.tie(0);
#define int long long
template<class T>void chmax(T &a, T b){if (a < b)a = b;}
template<class T>void chmin(T &a, T b){if (b < a)a = b;}
using namespace std;
const int MAXN = (int)2e5 + 5;
const int INF = 1e18;
int a[MAXN];
int suf[MAXN], pref[MAXN];
signed main() {
fastInp;
int n;
cin >> n;
for (int i = 1; i <= n; i++) {
cin >> a[i];
}
for (int i = n - 1; i >= 1; i--) {
suf[i] = suf[i + 1] + max(0ll, a[i + 1] - a[i] + 1);
}
int ans = suf[1];
for (int i = 2; i < n; i++){
pref[i] = pref[i - 1] + max(0ll, a[i - 1] - a[i] + 1);
ans = min(ans, max(pref[i], suf[i]));
}
pref[n] = pref[n - 1] + max(a[n - 1] - a[n] + 1, 0ll);
ans = min(ans, pref[n]);
cout << ans << endl;
}
/*
8
12 2 34 85 4 91 29 85
*/
