#include <bits/stdc++.h>
using namespace std;
void solve() {
  int n;
  cin >> n;
  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 - 1] > a[i]) {
      sum += (a[i - 1] - a[i]);
      a[i] = a[i - 1];
    }
  }
  cout << sum << endl;
}
int main() {
  int t;
  t = 1;
  while (t--) {
    solve();
  }
}
