#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  ;
  int t = 1;
  while (t--) {
    int n;
    cin >> n;
    vector<int> v(n);
    for (int i = 0; i < n; i++) {
      cin >> v[i];
    }
    int curr = v[0], ans = 0;
    for (int i = 1; i < n; i++) {
      if (v[i] <= curr)
        ans += curr - v[i];
      else
        curr = v[i];
    }
    cout << ans << "\n";
  }
}
