#include <bits/stdc++.h>
using namespace std;
long long gcd(long long a, long long b) { return (a ? gcd(b % a, a) : b); }
long long power(long long a, long long n) {
  long long p = 1;
  while (n > 0) {
    if (n & 1) {
      p = p * a;
    }
    n >>= 1;
    a *= a;
  }
  return p;
}
long long power(long long a, long long n, long long mod) {
  long long p = 1;
  while (n > 0) {
    if (n & 1) {
      p = p * a;
      p %= mod;
    }
    n >>= 1;
    a *= a;
    a %= mod;
  }
  return p % mod;
}
template <typename T>
ostream& operator<<(ostream& os, const vector<T>& p) {
  os << "[ ";
  for (T x : p) os << x << " ";
  os << "]" << endl;
  return os;
}
template <typename T>
ostream& operator<<(ostream& os, const set<T>& p) {
  os << "{ ";
  for (T x : p) os << x << " ";
  os << "}" << endl;
  return os;
}
template <typename T>
ostream& operator<<(ostream& os, const multiset<T>& p) {
  os << "{ ";
  for (T x : p) os << x << " ";
  os << "}" << endl;
  return os;
}
template <typename Tk, typename Tv>
ostream& operator<<(ostream& os, const map<Tk, Tv>& p) {
  os << "{ ";
  for (pair<Tk, Tv> x : p) os << x << " ";
  os << "}" << endl;
  return os;
}
template <typename Tk, typename Tv>
ostream& operator<<(ostream& os, const pair<Tk, Tv>& p) {
  os << "{" << p.first << ',' << p.second << "}";
  return os;
}
const int N = 100005;
int m[N], t[N];
int main() {
  int i, j;
  int n;
  scanf("%d", &n);
  for (i = 1; i < n + 1; i++) scanf("%d", &m[i]);
  for (i = 1; i < n + 1; i++) t[i] = max(t[i - 1], m[i] + 1);
  int x = t[n];
  for (i = n - 1; i >= 1; i--) {
    x--;
    t[i] = max(t[i], x);
    x = max(x, t[i]);
  }
  long long ans = 0;
  for (i = 1; i < n + 1; i++) ans += t[i] - m[i] - 1;
  printf("%lld\n", (long long)ans);
  return 0;
}
