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