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