#include <bits/stdc++.h>
using namespace std;
const long long N = 1e5 + 5;
long long v[N], mn[N];
int main() {
  long long n;
  long long ans = 0;
  cin >> n;
  for (long long i = 0; i < n; i++) cin >> v[i];
  for (long long i = 0; i < n; i++) mn[i] = v[i] + 1;
  for (long long i = 1; i < n; i++) mn[i] = max(mn[i - 1], mn[i]);
  for (long long i = n - 2; i >= 0; i--) mn[i] = max(mn[i], mn[i + 1] - 1);
  for (long long i = 0; i < n; i++) ans += mn[i] - v[i] - 1;
  cout << ans << endl;
}
