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