#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using P = pair<int, int>;
int main() {
  int n;
  cin >> n;
  vector<int> a(n);
  for (int i = 0; i < (n); i++) {
    cin >> a.at(i);
  }
  int min = 0;
  for (int i = 0; i < (n - 1); i++) {
    if (a.at(i) > a.at(i + 1)) {
      while (a.at(i) > a.at(i + 1)) {
        a.at(i + 1)++;
        min++;
      }
    }
  }
  cout << min << endl;
  return 0;
}
