#include <bits/stdc++.h>
using namespace std;
const long long mod = 998244353;
const int maxn = 1e5 + 5;
const int inf = 0x3f3f3f3f;
const double PI = acos(-1.0);
const long long INF = 0x3f3f3f3f3f3f3f3f;
int a[maxn], val[maxn];
int main() {
  int n;
  cin >> n;
  for (int i = 1; i <= n; ++i) {
    cin >> a[i];
    val[i] = a[i] + 1;
  }
  for (int i = 1; i <= n; ++i) {
    val[i] = max(val[i], val[i - 1]);
  }
  for (int i = n; i > 0; --i) {
    val[i] = max(val[i + 1] - 1, val[i]);
  }
  long long ans = 0;
  for (int i = 1; i <= n; ++i) {
    ans += val[i] - a[i] - 1;
  }
  cout << ans << endl;
  return 0;
}
