#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  int n;
  cin >> n;
  vector<int> a(n);
  for (int i = 0; i < n; i++) {
    cin >> a[i];
  }
  int sum = 0;
  for (int i = 1; i < n; i++) {
    if (a[i] < a[i - 1]) {
      int diff = a[i - 1] - a[i];
      sum += diff;
      a[i] += diff;
    }
  }
  cout << sum;
  return 0;
}
