#include <bits/stdc++.h>
using namespace std;
long long int md = 1e9 + 7;
long long int __gcd(long long int a, long long int b) {
  if (b == 0) return a;
  return __gcd(b, a % b);
}
long long int poww(long long int a, long long int b, long long int md) {
  if (b < 0) return 0;
  if (a == 0) return 0;
  long long int res = 1;
  while (b) {
    if (b & 1) {
      res = (1ll * res * a) % md;
    }
    a = (1ll * a * a) % md;
    b >>= 1;
  }
  return res;
}
long long int poww(long long int a, long long int b) {
  if (b < 0) return 0;
  if (a == 0) return 0;
  long long int res = 1;
  while (b) {
    if (b & 1) {
      res = (1ll * res * a);
    }
    a = (1ll * a * a);
    b >>= 1;
  }
  return res;
}
void ainp(long long int arr[], long long int n) {
  for (long long int i = 1; i <= n; i++) cin >> arr[i];
}
long long int divide(long long int a, long long int b, long long int md) {
  long long int rr = a * (poww(b, md - 2, md));
  rr %= md;
  return rr;
}
const long long int size = 1e5 + 1;
long long int n;
long long int arr[size], total[size];
signed main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  cin >> n;
  long long int sum = 0, ans = 0;
  for (long long int i = 1; i <= n; i++) cin >> arr[i];
  for (long long int i = 1; i <= n; i++)
    total[i] = max(total[i - 1], arr[i] + 1);
  for (long long int i = n - 1; i > 0; i--)
    total[i] = max(total[i + 1] - 1, total[i]), ans += total[i] - 1 - arr[i];
  ans += total[n] - 1 - arr[n];
  cout << ans << "\n";
  return 0;
}
