#include <bits/stdc++.h>
using namespace std;
int a[200010];
int main() {
  ios_base::sync_with_stdio(0);
  int n;
  cin >> n;
  for (int i = 0; i < n; i++) cin >> a[i];
  long long ans = 0;
  for (int i = 1; i < n; i++) {
    int diff = max(0, a[i] - a[i - 1]);
    ans += diff;
    a[i] += diff;
  }
  cout << ans << endl;
}
