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