#include <bits/stdc++.h>
using namespace std;
int n;
long long stool = 0;
int main() {
  cin >> n;
  int arr[n];
  for (int i = 0; i < n; i++) {
    cin >> arr[i];
  }
  for (int i = 1; i < n; i++) {
    if (arr[i] >= arr[i - 1]) {
      continue;
    } else {
      stool += arr[i - 1] - arr[i];
      arr[i] += 1;
    }
  }
  cout << stool;
  return 0;
}
