#include <bits/stdc++.h>
using namespace std;
const int maxn = 2e5 + 7;
const int inf = 0x3f3f3f3f;
const int mod = 1e9 + 7;
long long ans;
int a[maxn];
int ned[maxn];
int n;
int main() {
  scanf("%d", &n);
  for (int i = 1; i <= n; i++) {
    scanf("%d", &a[i]);
    ned[i] = a[i] + 1;
  }
  for (int i = 2; i <= n; i++) {
    ned[i] = max(ned[i], ned[i - 1]);
  }
  for (int i = n; i >= 1; i--) {
    ned[i] = max(ned[i + 1] - 1, ned[i]);
    ans += (ned[i] - a[i] - 1);
  }
  printf("%I64d\n", ans);
}
