#include <bits/stdc++.h>
using namespace std;
const int N = 1e5 + 1;
int mark[N], t[N], n;
long long res;
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  ;
  cin >> n;
  for (int i = (1); i <= (n); i += (1)) cin >> mark[i];
  int cur = 0;
  for (int i = (n); i >= (1); i -= (1)) {
    cur = max(0, cur - 1);
    cur = max(cur, mark[i] + 1);
    t[i] = cur;
  }
  cur = 0;
  for (int i = (1); i <= (n); i += (1)) {
    cur = max(cur, t[i]);
    res += cur;
  }
  for (int i = (1); i <= (n); i += (1)) res -= (mark[i] + 1);
  cout << res << "\n";
  return 0;
}
