#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  int a[n];
  for (int i = 0; i < (n); ++i) cin >> a[i];
  int mx = -1, ans = 0;
  for (int i = 0; i < (n); ++i) {
    mx = max(mx, a[i]);
    ans += max(mx - a[i], 0);
  }
  cout << ans << endl;
  return 0;
}
