#include <bits/stdc++.h>
using namespace std;
#define forsn(i, s, n) for (int i = int(s); i < int(n); i++)
#define forn(i, n) forsn(i, 0, n)
#define dforsn(i, s, n) for (int i = int(n) - 1; i >= int(s); i--)
#define dforn(i, n) dforsn(i, 0, n)
#define sz(x) int(x.size())
#define all(x) begin(x), end(x)
using ll = long long;
using ld = long double;
using vi = vector<int>;
using vb = vector<bool>;
using vll = vector<ll>;
using ii = pair<int, int>;
using iii = tuple<int, int, int>;
#define fst first
#define snd second
#define pb push_back
#define eb emplace_back
const ll INF = 1e18;
int main() {
ios::sync_with_stdio(0);
cin.tie(0); cout.tie(0);
int n;
cin >> n;
vi a(n);
forn(i, n) cin >> a[i];
vi d(n - 1);
forn(i, n - 1) d[i] = a[i + 1] - a[i];
vll pref(n), suff(n);
pref[0] = 0LL;
forn(i, n - 1) {
pref[i + 1] = pref[i] + max(0, 1 - d[i]);
}
suff[n - 1] = 0LL;
dforn(i, n - 1) {
suff[i] = suff[i + 1] + max(0, d[i] + 1);
}
ll ret = INF;
forn(i, n) ret = min(ret, max(pref[i], suff[i]));
cout << ret << '\n';
return 0;
}
