#include <bits/stdc++.h>
using namespace std;
long long n, a[1231231];
long long mark[1231231];
int main() {
  cin >> n;
  for (int i = 1; i <= n; i++) cin >> a[i];
  mark[1] = 1;
  for (int i = 2; i <= n; i++) {
    mark[i] = mark[i - 1];
    mark[i] = max(mark[i], a[i] + 1);
  }
  for (int i = n - 1; i >= 1; i--) {
    mark[i] = max(mark[i], mark[i + 1] - 1);
  }
  long long ans = 0;
  for (int i = 1; i <= n; i++) {
    ans += max(0ll, (mark[i] - 1 - a[i]));
  }
  cout << ans << endl;
}
