#include <bits/stdc++.h>
using namespace std;
using ll = long long;
template <class T>
using Grid = vector<vector<T>>;
int main() {
  int n;
  cin >> n;
  vector<int> v(n);
  for (int i = 0; i < (n); i++) cin >> v[i];
  int sum = 0;
  for (int i = 1; i < n; i++) {
    if (v[i - 1] > v[i]) {
      sum += v[i - 1] - v[i];
      v[i] = v[i - 1];
    }
  }
  cout << sum << endl;
}
