#include <bits/stdc++.h>
#include <experimental/random>
#include <random>
using namespace std;
using ll = long long;
using ld = long double;
const ll INF = 1e18, MOD = 998244353;
void solve();
signed main() {
#ifdef LOCAL
freopen("input.txt", "r", stdin);
freopen("output.txt", "w", stdout);
#endif
ios_base::sync_with_stdio(false);
cin.tie(0);
cout.tie(0);
int q = 1;
//cin >> q;
while (q--) {
solve();
}
}
void solve() {
ll n;
cin >> n;
vector<ll> a(n);
for (int i = 0; i < n; i++) {
cin >> a[i];
}
vector<ll> pref(n);
for (int i = 1; i < n; i++) {
pref[i] = pref[i - 1] + max(0ll, a[i - 1] - a[i] + 1);
}
vector<ll> suf(n);
for (int i = n - 2; i >= 0; i--) {
suf[i] = suf[i + 1] + max(0ll, a[i + 1] - a[i] + 1);
}
ll ans = INF;
for (int i = 0; i < n; i++) {
ans = min(ans, max(pref[i], suf[i]));
}
cout << ans;
}
