#include <bits/stdc++.h>
using namespace std;
int main(void) {
  int n;
  cin >> n;
  vector<int> a(n);
  for (int i = 0; i < n; i++) {
    cin >> a[i];
  }
  int num = 0;
  for (int i = 1; i < n; i++) {
    if (a[i] >= a[i - 1]) {
      continue;
    }
    while (a[i] < a[i - 1]) {
      a[i]++;
      num++;
    }
  }
  cout << num << endl;
}
