#include <bits/stdc++.h>
using namespace std;
const int N = 100100;
long long int a[N], l[N];
int main() {
  int n;
  long long int ans(0);
  scanf("%d", &n);
  for (int i = 1; i <= n; ++i) scanf("%lld", &a[i]), a[i]++;
  long long int o(a[n] + 1);
  for (int i = n; i >= 1; --i) l[i] = o = max(--o, a[i]);
  for (int i = 1; i <= n; ++i) l[i] = max(l[i], l[i - 1]);
  for (int i = 1; i <= n; ++i) ans += (l[i] - a[i]);
  printf("%lld", ans);
  return 0;
}
