#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  int TESTS = 1;
  while (TESTS--) {
    long long n;
    cin >> n;
    vector<long long int> a(n);
    for (long long int i = 0; i < n; i++) cin >> a[i];
    vector<long long int> t(n);
    t[0] = 1;
    for (long long int i = 1; i < n; i++) t[i] = max(a[i] + 1, t[i - 1]);
    for (int i = n - 2; i >= 0; i--) t[i] = max(t[i + 1] - 1, t[i]);
    long long s = 0;
    for (long long int i = 0; i < n; i++) s += t[i] - a[i] - 1;
    cout << s;
  }
  return 0;
}
