#include <bits/stdc++.h>
using namespace std;
const long long inf = 4e18;
const long long N = 2e5 + 10;
void testCase() {
  long long n;
  cin >> n;
  long long a[n + 1];
  for (long long i = 1; i <= n; i++) cin >> a[i];
  vector<long long> sma(n + 2), suf(n + 2);
  for (long long i = n; i >= 1; i--) {
    sma[i] = max(a[i], sma[i + 1] + 1);
    suf[i] = suf[i + 1] + sma[i] - a[i];
  }
  long long ans = inf;
  long long pma = 0, pref = 0;
  for (long long i = 1; i <= n; i++) {
    pma = max(pma + 1, a[i]);
    long long cur = pref + suf[i + 1] + max(pma, sma[i]) - a[i];
    ans = min(ans, cur);
    pref += pma - a[i];
  }
  cout << ans;
}
int32_t main() {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr), cout.tie(nullptr);
  long long t_c = 1;
  while (t_c--) testCase();
}
