#include <bits/stdc++.h>
using namespace std;
int32_t main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int t = 1;
  while (t--) {
    int n, ans = 0;
    cin >> n;
    int a[n];
    for (int i = 0; i < n; i++) {
      cin >> a[i];
    }
    for (int i = 1; i < n; i++) {
      if (a[i] < a[i - 1]) {
        ans += a[i - 1] - a[i];
        a[i] = a[i - 1];
      }
    }
    cout << ans;
  }
  return 0;
}
