#include <bits/stdc++.h>
using namespace std;
using namespace std::chrono;
inline long long lrand() {
  long long x = 0;
  for (int i = 0; i < (4); i++) {
    x <<= 16;
    x ^= rand();
  }
  return x;
}
string fname(string s, vector<long long> v) {
  string x = "C:/leet/" + s;
  for (auto& k : (v)) x += "_" + to_string(k);
  x += ".txt";
  return x;
}
long long now() {
  return duration_cast<milliseconds>(system_clock::now().time_since_epoch())
      .count();
}
template <typename T>
void W(vector<T>& v) {
  for (int i = 0; i < (v.size()); i++) cout << v[i] << ' ';
}
int main(int argc, char** argv) {
  cin.sync_with_stdio(false);
  long long n;
  scanf("%lld", &n);
  vector<long long> a(100007, 0), x(100007, 0);
  for (int i = (1); i <= (n); i++) scanf("%lld", &a[i]);
  for (int i = (1); i <= (n); i++) {
    x[i] = x[i - 1];
    x[i] = max(x[i], a[i] + 1);
  }
  for (int i = n - 1; i >= 1; i--) {
    x[i] = max(x[i], x[i + 1] - 1);
  }
  long long sum = 0;
  for (int i = 1; i <= n; i++) {
    sum += x[i] - a[i] - 1;
  }
  printf("%lld\n", sum);
  return 0;
}
