#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
/**
* JOI 2021 Final Round - Biba-herbs
* Vaqt murakkabligi: O(N)
*/
int main() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
int N;
cin >> N;
vector<long long> A(N + 1);
for (int i = 1; i <= N; i++) {
cin >> A[i];
}
if (N == 1) {
cout << 0 << endl;
return 0;
}
vector<long long> diff(N);
for (int i = 1; i < N; i++) {
diff[i] = A[i + 1] - A[i];
}
// pref[i] - 1-dan i-gacha o'suvchi qilish uchun kerakli amallar
vector<long long> pref(N + 1, 0);
for (int i = 1; i < N; i++) {
long long cost = 0;
if (diff[i] <= 0) {
cost = abs(diff[i]) + 1;
}
pref[i + 1] = pref[i] + cost;
}
// suff[i] - i-dan N-gacha kamayuvchi qilish uchun kerakli amallar
vector<long long> suff(N + 1, 0);
for (int i = N - 1; i >= 1; i--) {
long long cost = 0;
if (diff[i] >= 0) {
cost = diff[i] + 1;
}
suff[i] = suff[i + 1] + cost;
}
long long min_ops = -1;
for (int k = 1; k <= N; k++) {
// k cho'qqi bo'lganda kerakli sug'orishlar soni
long long current_ops = max(pref[k], suff[k]);
if (min_ops == -1 || current_ops < min_ops) {
min_ops = current_ops;
}
}
cout << min_ops << endl;
return 0;
}
