#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 stool = 0;
  for (int i = 1; i < n; i++) {
    if (a[i - 1] > a[i]) {
      stool += a[i - 1] - a[i];
      a[i] += a[i - 1] - a[i];
    }
  }
  cout << stool;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  solve();
  return 0;
}
