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