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