#include <bits/stdc++.h>
using namespace std;
#define FOR(i, a, b) for (int i = (a), _b = (b); i <= _b; ++i)
#define FORD(i, a, b) for (int i = (a), _b = (b); i >= _b; --i)
using ll = long long;
const int MAXN = 2e5 + 10;
int n;
ll a[MAXN];
ll pref[MAXN], suff[MAXN];
int main() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
cin >> n;
FOR(i, 1, n) cin >> a[i];
// pref[i]: min ops to make a[1..i] strictly increasing
ll cur = a[1];
ll prev_add = 0; // cur - a[i] at previous position
pref[1] = 0;
FOR(i, 2, n) {
cur = max(cur + 1, a[i]);
ll add = cur - a[i];
pref[i] = pref[i - 1] + max(0LL, add - prev_add);
prev_add = add;
}
// suff[i]: min ops to make a[i..n] strictly decreasing
cur = a[n];
prev_add = 0;
suff[n] = 0;
FORD(i, n - 1, 1) {
cur = max(cur + 1, a[i]);
ll add = cur - a[i];
suff[i] = suff[i + 1] + max(0LL, add - prev_add);
prev_add = add;
}
ll res = (ll)4e18;
FOR(i, 1, n) {
res = min(res, max(pref[i], suff[i]));
}
cout << res << '\n';
return 0;
}
