#include <bits/stdc++.h>
using namespace std;
signed main() {
  cin.tie(0);
  cout.tie(0);
  cin.sync_with_stdio(0);
  cout.sync_with_stdio(0);
  long long n;
  cin >> n;
  vector<long long> v(n);
  vector<long long> msuf(n + 1);
  for (long long i = 0; i < n; ++i) cin >> v[i];
  for (long long i = n - 1; i >= 0; --i)
    msuf[i] = max(v[i] + 1, msuf[i + 1] - 1);
  for (long long i = 1; i < n; ++i) msuf[i] = max(msuf[i], msuf[i - 1]);
  long long second = 0;
  for (long long i = 0; i < n; ++i) second += msuf[i] - v[i] - 1;
  cout << second << endl;
  return 0;
}
